feat: Add upm target

[bitsandfoxes]

Unity SDK works in two repositories:

https://github.com/getsentry/sentry-unity <- SDK
https://github.com/getsentry/unity <- release repo for the user
Unity users add our package by providing a git URL to their package manager. Versioning and potential rollbacks for them are made available via tags (i.e. https://github.com/getsentry/unity.git#x.x.x).

Release steps:

1. Get the prepared artifact (a single .zip containing the package)
2. Pull the release repository
3. Replace everything in that repo with the contents of the .zip
4. Update tag & push

[tonyo]

I see this is a Draft, but just a few things not to forget about:

• Let's update the README with the docs about the new target
• Let's make sure that tests and linters are green. @BYK what's the linting story right now? Do we use eslint? prettier? Also, I'm not sure we're running them as part of CI now.

[bruno-garcia]

Looking good, maybe drop the draft tag?

[BYK]

Added the lint automation and fixes here: #214

[BYK]

I think these relative imports will fix the test issues. I'll look into why this is breaking separately.

Btw is it possible to have some tests for this target?

[lobsterkatie]

Mostly just proofreading. (You can take the girl out of copyediting...) Otherwise LGTM!

[bruno-garcia]

Thanks a lot @BYK , @bitsandfoxes , @tonyo , @lobsterkatie !!!

🥳 🥳