chore: Add support for running tests & prettier in CI

[BenHenning]

This PR:

1. Adds support for both fixing eslint issues and running prettier locally (for parity with core Blockly).
2. Isolates the lint check with a general build check in CI.
3. Adds a prettier format check.
4. Updates the build/lint CI workflows to also run on a PR being merged into main.
5. Adds a workflow for running the new WebdriverIO tests for both PRs and after being merged into main. These run on both OS X and Ubuntu latest, and run with failfast turned off (so that OS-specific failures can be seen).
6. Sets up the build, lint, and WebdriverIO tests to run against latest v12 tip-of-tree so that CI workflows don't fail when core is updated. This is assuming that we'll generally change core before the plugin. It can introduce some spurious failures during plugin development, but it should ensure eventual consistency.
7. Introduces separate build, lint, and test workflows for running against the latest released (pinned) v12 instead of tip-of-tree. These only run on the main branch and manually so that we can more easily check current compatibility (which can help determine if we need to release a new version of v12).

[BenHenning]

/cc @microbit-matt-hillsdon @maribethb and @rachel-fenichel since it came up in the meeting this morning.