Initial commit of override tooling

[NickGerleman]

Add a foundation for new override tooling described in #4104. This includes:

• Build scripts, lint scripts, config files, etc
• Logic for parsing and checking validity of an override manifest
• Unit tests for override manifest logic
• Abstractions to allow fetching React Native files of arbtrary versions

A lot of this is foundational. The override logic has been well-tested,
and the Git logic has been manually tested, but we don't have much
end-to-end set up yet.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

@acoates-ms do we have a nice way to hook in jest tests for a package into CI? Having tests locally only for just this isn't too bad, but if it's easy, I would rather add JS tests to there as well.

[acoates-ms]

We should introduce a new top level command for "test", which would run UTs.
Should be easy to add a new command to package.json that runs lerna to run the test command on all the packages.

Then pick one of the CI slices to trigger the test command. (I generally look at which one is currently quickest). -- you'll want to make sure its one of the ones that has already done yarn build.

.. Then only other tricky part is that it looks like E2E test package is already using the test script for something. ... So then I guess either pick a different name than test either for the new task, or rename the existing E2E one, which probably also means modifying the CI loop.

[NikoAri]

[vmoroz]

[NickGerleman]

We should introduce a new top level command for "test", which would run UTs.
Should be easy to add a new command to package.json that runs lerna to run the test command on all the packages.

Then pick one of the CI slices to trigger the test command. (I generally look at which one is currently quickest). -- you'll want to make sure its one of the ones that has already done yarn build.

.. Then only other tricky part is that it looks like E2E test package is already using the test script for something. ... So then I guess either pick a different name than test either for the new task, or rename the existing E2E one, which probably also means modifying the CI loop.

Looks like using the just-task template actually puts unit tests as part of the build. This seems reasonable, since on my machine it takes 5s to run tests without the jest cache. That should get us CI coverage without any extra work.

[ghost]

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