Rework action inner workings (and move to v2)

[KyleMayes]

This reworks this action to pull LLVM and Clang binaries from a new assets.json file that maps operating system and architecture (x86 or arm64) to the supported LLVM and Clang versions (and their associated asset paths) for that operating system and architecture.

This assets.json file is generated by the new generate.ts file which uses the GitHub API to query the releases for the llvm/llvm-project and extract supported assets. This should reduce the manual effort in adding support for future LLVM and Clang versions.

As documented in the new CHANGELOG.md file:

Migrating from v1

• Support for LLVM and Clang 3.5 through 7.0 has been removed (use 7.1 or later)
• The download-url input has been renamed to mirror-url
• The force-version and ubuntu-version inputs have been replaced with the force-url input

Other Changes

• Added arch input
• Added support for various missing LLVM and Clang versions up to 18.1.2