test(test-repos): Add a test monorepo under /test-repos/yarn-pnpm

[JasonVMo]

Description

There are several tools built in rnx-kit that it would be beneficial to test in a separate repo. This creates a separate monorepo, set up with yarn in pnpm mode under /test-repos/yarn-pnpm. This will allow us to leverage tools built in rnx-kit and test them out in various configurations.

• In particular this is essential for being able to test the yarn plugin for external workspaces currently active as a draft PR.

Currently this test repo isn't included in CI loops, as it is pulling in rnx-kit packages via npm, but that will be added once it is in a nice state and is actually testing out some of the tools directly. Ideally it will reference the rnx-kit packages via the external workspaces plugin.

For the repo itself:

• it has an eslint-config package, two controls copied and adapted from FURN (button and text), then the test-app code split into three packages, test-app-shared, test-app-desktop, and test-app-mobile.
• scripts is set up in much the same fashion as for rnx-kit itself.
• yarn is in pnpm mode
• right now only the build/lint scripts are working. The bundle commands route to the rnx-cli correctly but I haven't worked through the issues.
• It uses lage for builds rather than nx since it let's us test the tool that is more common internally.

[tido64]

Can we use portal: instead of copying these two packages?

[JasonVMo]

The scripts will be not quite the same but I can make that adjustment for the eslint-config package.

[tido64]

The package folder should be all lower-case.