Vastly overhaul the CI workflow for build checks

This workflow compiles TFQ and runs test cases to verify things work after file changes are made by a PR or other event. It is a complete rewrite of the previous TFQ build & test checks CI workflow, and incorporates many new features and efficiency gains:

- Serious use of caching of the Python installation and environment, as well as the Bazel environment, so that repeated runs don't have to reinstall everything every time.

- File-aware execution: the workflow tests whether source files and build configuration files have been modified in a given event, and stops early if the event didn't affect such files.

- Compatibility with GitHub [merge queues](https://github.blog/news-insights/product-news/github-merge-queue-is-generally-available/).

- Concurrency detection: if a new event such as a push to a PR branch comes in while a workflow is already executing, that workflow is cancelled and a new one restarted. (The existing workflow keeps the existing one running and starts a second one, which is almost never useful.)

- Addition of a manual run interface, allowing the workflow to be invoked manually from the GitHub GUI with different values for program versions – useful for debugging, testing, exploring behaviors with different versions of linters, etc.

Author: mhucka