No error for missing directives `=`

[babakfp]

Describe the bug

Hi
There was a typo in my code and I didn't notice it for a while. Svelte needs to show an error if the = was missing. Please check the REPL.
Thanks

Reproduction

https://svelte.dev/repl/ef6b6fb2c44e4bf3b2ce656641676985?version=3.43.1

Logs

No response

System Info

System:
    OS: Windows 10 10.0.22000
    CPU: (12) x64 Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
    Memory: 7.85 GB / 15.84 GB
  Binaries:
    Node: 14.17.6 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.15 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.14.15 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.22000.120.0), Chromium (94.0.992.38)
    Internet Explorer: 11.0.22000.120
  npmPackages:
    rollup: ^2.3.4 => 2.58.0
    svelte: ^3.0.0 => 3.43.1

Severity

blocking all usage of svelte

[Conduitry]

on:message{handleMessage} is technically valid. It means that message{handleMessage} events will be passed on to the parent component. One of the things considered for Svelte 4 is to have more strict rules around the characters allowed in event names.

[dummdidumm]

I've seen some pretty strange event names in the wild, partially to work around the current Svelte limitation that you can't bubble all events at once, so I'm not sure what we can do here.
Slightly related to #6823 which is also about restricting things.