Add C example and file extension.

[agoessling]

This adds clang-format support for C files *.c. clang-tidy support already works correctly for cc_* targets with C sources.

It wasn't clear the best way to add *.c to format.sh given how it seems the output of mirror_linguist_languages.sh is manually copied over.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
  • Given how cc_* targets silently handle C and C++ users likely already expect C support.
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes
  • Add support for formatting C files.

Test plan

• New test cases added

[CLAassistant]

All committers have signed the CLA.

[alexeagle]

I think the more principled answer is to add the missing language: https://github.com/github-linguist/linguist/blob/559a6426942abcae16b6d6b328147476432bf6cb/lib/linguist/languages.yml#L720-L734

We have other cases where one formatter covers multiple languages.

[agoessling]

@alexeagle That does sound better. Can you point me to an example to pattern match from?

[alexeagle]

I think it's simply following the pattern that C++ and other languages here already use.

[agoessling]

Ok, I think this is a more principled way of adding C formatting support.

[alexeagle]

excellent! thanks a bunch!