fix: update app event filter example to be a single 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 Filter 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-filter.

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 filter function and a script to create a content type with the correct fields and publish two entries of differing sentiment
• Bumped @contentful/node-apps-toolkit and use the FunctionTypeEnum

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

Dependencies and/or References

Deployment

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[michaelpineirocontentful]

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