Make the on modifier built-in in strict-mode

text/0997-make-on-built-in.md

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+---
+stage: accepted
+start-date: 2023-12-22T00:00:00.000Z
+release-date: # In format YYYY-MM-DDT00:00:00.000Z
+release-versions:
+teams: # delete teams that aren't relevant
+  - framework
+  - learning
+  - typescript
+prs:
+  accepted: https://github.com/emberjs/rfcs/pull/997
+project-link:
+suite: 
+---
+
+<!--- 
+Directions for above: 
+
+stage: Leave as is
+start-date: Fill in with today's date, 2032-12-01T00:00:00.000Z
+release-date: Leave as is
+release-versions: Leave as is
+teams: Include only the [team(s)](README.md#relevant-teams) for which this RFC applies
+prs:
+  accepted: Fill this in with the URL for the Proposal RFC PR
+project-link: Leave as is
+suite: Leave as is
+-->
+
+# Make `{{on}}` a built in modifier  
+
+## Summary
+
+Today, when using gjs/gts/`<template>`, in order to bind event listeners, folks _must import_ the `{{on}}` modifier.
+Because event listening is so commonplace, this is a grating annoyance for developers.
+
+This RFC proposes that `{{on}}` be built in to `glimmer-vm` and not require importing.
+
+## Motivation
+
+Given how common it is to use the `{{on}}` modifier:
+
+```gjs
+import { on } from '@ember/modifier';
+
+<template>
+    <button {{on 'click' @doSomething}}>
+        click me
+    </button>
+
+    <form {{on 'submit' @localSubmit}}>
+        <label
+            {{on 'keydown' @a}}
+            {{on 'keyup' @a}}
+            {{on 'focus' @a}}
+            {{on 'blur' @a}}
+        >
+        </label>
+
+        <button>
+            submit
+        </button>
+    </form>
+</template>
+```
+
+It should be built in to the templating engine, Glimmer, so that folks don't need to import it.
+
+There is precedence for this already as the following are already commonplace and built in:
+- `(helper)`
+- `(modifier)`
+- `(component)`
+
+It's historically been the stance that, 
+
+"If it can be built in userspace, it should be, leaving the framework-y things to be only what can't exist in userspace"
+
+But to achieve the ergonomics that our users want, we should provide a more cohesive experience, rather than require folks import from all of (in the same file):
+- `@glimmer/component`
+- `@glimmer/tracking`
+- `@ember/modifier`
+- `@ember/helper`
+- `ember-modifier`
+- `ember-resources`
+- `tracked-built-ins`
+
+Some of the above may unify in a separate RFC, but for the template-utilities, since the modules that provide them are already so small, it makes sense to inherently provide them by default. Especially as we can target strict-mode only, so that we don't run in to the same implementation struggles that built-in [Logical Operators](https://github.com/emberjs/rfcs/pull/562), [Numeric Comparison Operators](https://github.com/emberjs/rfcs/pull/561), and [Equality Operators](https://github.com/emberjs/rfcs/pull/560) have faced.
+
+<details><summary>some context on those RFCs</summary>
+
+The main problem with adding default utilities without strict-mode is that it becomes very hard to implement a way for an app to incrementally, and possibly per-addon, or per-file, to adopt the default thing due to how resolution works. Every usage of the built in utility would also require a global resolution lookup (the default behavior in loose mode templates) to see if an addon is overriding the built ins -- and then, how do you opt in to the built ins, and _not_ let addons override what you want to use?
+
+With gjs/gts/`<template>`, this is much simpler, as in strict-mode, you can check if the scope object defines the helpers, and if not, use the built in ones.
+
+This strategy of always allowing local scope to override default-provided utilities will be a recurring theme.
+
+</details>
+
+---------------
+
+_Making `on` a built-in will help make writing components feel more cohesive and well supported, as folks will not need to cobble together many imported values_
+
+----------------
+
+## Detailed design
+
+This change would affect strict-mode only. This is so that today's existing code that imports `on` from `@ember/modifier` will still work due to how values defined locally in scope override globals.
+
+The behavior of `on` would be the same as it is today, but defined by default in the `glimmer-vm`.
+
+
+`on` will be a keyword, and for backwards compatibility, this will require that keywords, in strict mode, be overrideable by the strict-mode scope bag.
+
+
+## How we teach this
+
+Once implemented, the guides, if they say anything about gjs/gts/`<template>` and `on` by the time this would be implemented, would only remove the import.
+
+## Drawbacks
+
+People may not know where `on` is defined.
+- counterpoint: do they need to?, we are defining a lanugage, trying to make it ergonomic.
+
+## Alternatives
+
+- Use a prelude
+    - preludes were mentioned during the initial exploration of strict-mode templates, and were decided against, because addons would not be able to assume a prelude exists, as apps could define their own, and this sort of re-introduces the app-tree-merging behavior that we've been trying to get away from. 
+
+- Use an alternate syntax: `on:click={{handler}}` or `on:{eventname}={{value}}`
+    - This would be even more ergonomic, and I think we should do this syntax anyway, but may take longer to implement. -- thought would not require glimmer allow the scope bag to overrid keywords.
+
+
+## Unresolved questions
+
+- What happens if we want to remove a keyword? (like `mut`)
+  - same as today, we only need to commit to a major to remove the keyword in and then do it - providing ample deprecation time, ending with the final removal.