Setup clang format for C code

[st0012]

While working on #2398, we found it necessary to have some sort of auto-formatting for C code as well to ensure consistent coding style. But instead of adding it as part of #2398, which already introduces many changes on the implementation itself, I think it'd be better to have the auto-formatting setup in a separate PR.

This PR:

• Sets up clang-format for the project, including
  • .clang-format defines the rules of formatting (like rubocop.yml)
  • A new format:c rake task will perform the formatting to all C code under ext/, src/, and include/
  • A new format:c_check rake task will check if formatting is required without actually making the changes
  • A new CI workflow to perform format:c_check
  • Introductions about C formatting in the readme
• Sets up auto-formatting in VS Code, including:
  • Extension recommendation for the formatting
  • Workspace settings to enable it
• Formats the code with the given rules. The most noticeable changes are indentation related. Currently we have both 2-space and 4-space based indentation. Since the majority of the code uses 2-space indentation, which is what Introduce standalone C parser for RBS with arena allocation #2398 mostly follows, I set the rule to 2 and update the code that current indents with 4 spaces

[soutaro]

I'm good for clang-format. Can you add a commit for 4-space indentation?

[st0012]

Updated the indent width to 4 👍

[st0012]

@soutaro I decided to move all checking related tasks to the new c-check workflow so we can simplify the ruby builds too. WDYT?

[st0012]

We have to explicitly list many rules here to reduce/avoid differences between clang-format versions, which is hard to enforce across dev machines and CI.

[st0012]

@soutaro I think it's ready for review

[soutaro]

Thanks!

[soutaro]

Updated the branch protection setting to require format-check job.

[soutaro]

This step causes an error...
(Other jobs are fixed in #2485.)

[soutaro]

Merged this PR to continue working on other PRs.