build: remove package.json syncing

[hayemaxi]

Removes the need to copy/sync package JSON contents from core -> subproject extensions on build and package.

Summary of changes:

• Delete syncPackageJson.ts and handlePackageJson.ts
• Consolidate testing scripts and make small improvements (e.g. validation, handle multiple directories to import tests from)
• package.json is now fully complete in amazonq/, toolkit/. Changes to extensions should be made only the respective file (except for icons).
• toolkit/ runs unit, e2e, and integ tests from core/. It will also run any of its own tests (but those don't exist yet).
• Web tests now moved to and run from amazonq/. This was decided because amazonq/ is more likely to support web than toolkit/ at this time. Also, it didn't seem possible to run the web tests from core/ like the other tests because it relies on webpack and a separate test runner.
• No longer create web-based webpacks for core/

Follow up work:

• Get packageJSON properties from context.extension.packageJSON instead.
• Cleanup any remaining unneeded tasks in .vscode/tasks.json
• Move icons out of core/package.json.
• Continue to investigate: can we run web tests from core like other tests? Toolkit may have a use in web mode one day.

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License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[hayemaxi]

/runIntegrationTests

[justinmk3]

As a minor bonus, it looks like this also helps move in the direction of having a single Tests (current file) launch config, instead of 4+ configs for the same "current file" functionality.