feat(scripts): Allow jest tests to run multiple platforms, scripts cleanup

[JasonVMo]

Platforms Impacted

• test/script only change

Description of changes

This reworkes and cleans up the scripts/ package and allows jest tests to run on more than one platform. By default react-native tests will run on the platform specified by jestPlatform in the package.json (or 'ios') if not specified. But previously tests with a platform suffix like StackUtils.test.win32.ts wouldn't run because it wouldn't match the basic test file pattern.

Now the jest command will find all the test files in the folder, including those with module suffixes. If any module suffixes exist that differ from the default platform jest will run an additional pass against that target platform with those files selected.

This was originally part of a larger change which enables pinning tests for the existing components, including pinning/snapshotting across all relevant platforms.

Other changes

• removed the build and depcheck commands from scripts, they were not part of any validation flows
• removed depcheck references from package.json files.
• moved all usages of projRoot to the newer pkgContext implementation.

Verification

• automation