Clean up addons injection script and fix bugs in library

[agjohnson]

There is a lot here to modernize this library and fix a number of bugs:

• Sets this up as a package so we can use normal package management to define dependencies.
• Adds a test framework to help catch bugs. I also found this necessary just to understand what the worker was doing and verify changes.
• Reworks the docker container and image so that wrangler can gracefully catch file changes -- it's not necessary to constantly restart wrangler now.
• Switch worker from event based to ESM layout -- don't use fetch event and instead use fetch() exported function
• Alter how we are returning Response instances, and when this happens.

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• Fixes https://github.com/readthedocs/readthedocs.org/issues/11693
• Fixes https://github.com/readthedocs/addons/issues/410
• Refs https://github.com/readthedocs/readthedocs-ops/issues/1513
• Fixes #225
• Does not address #226 yet

[agjohnson]

I've tested this against projects with large response pages and the exception is actually caught now, and the response falls back to the origin response. The next change will be trying to catch this through streaming and avoiding buffering the entire request.