fix: update app event handler example to be frontendless app [EXT-6286]

[whitelisab]

Purpose

Since creating the new function templates, some of our function examples were not working properly due to the sub-directory setup. This PR changes the App Event Handler Function example to be a single frontendless app example that can be bootstrapped via the CCA using npx create-contentful-app@latest --example function-appevent-handler.

Approach

Here is a summary of the changes:

• Removed the TS/JS sub-directories in favor of a singular app example
• This example has no frontend; the only code is for functions and some setup scripts
• Add more details to the INSTRUCTIONS file, including how to use the scripts to easily get setup
• Added a script for creating an app event subscription that links to the handler function and a script to create a content type and publish an entry to trigger the events that will be handled and sent by the handler
• Bumped @contentful/node-apps-toolkit and use the FunctionTypeEnum

I also made two small updates to the filter example in this PR (updating one of the types and a docs update)

Testing steps

I tested this app locally and also via the CCA by pointing the local CCA to this branch. I ran through all the scripts in the instructions to ensure that they work properly.

Breaking Changes

None

Dependencies and/or References

Related PR for filter function example: #9602

Deployment

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