Add includes, flags, std, language, cppLinkStdLib options

[blaugold]

This PR adds a few more options to CBuilder that are commonly useful.
The flags option allows users to accomplish most things, even if CBuilder does support the specific use case.

The added options follow the example set by the cc crate.

[dcharkes]

LGTM w comments

Thanks @blaugold ! 🔧 🚀

[coveralls]

coverage: 98.099% (-0.09%) from 98.184% when pulling 8368aef on blaugold:includes-and-flags into 9f24b64 on dart-lang:main.

[dcharkes]

@blaugold looks like not all code is covered by tests https://coveralls.io/builds/62578519 (It doesn't show in the UI which lines unfortunately, but you can run coverage locally with dart pub global run coverage:test_with_coverage and use coverage gutters in VS code)

[dcharkes]

@blaugold looks like not all code is covered by tests https://coveralls.io/builds/62578519 (It doesn't show in the UI which lines unfortunately, but you can run coverage locally with dart pub global run coverage:test_with_coverage and use coverage gutters in VS code)

class Language {
  // ...

  @override
  String toString() => name; // not covered

[dcharkes]

lgtm!

[auto-submit]

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[dcharkes]

Thanks @blaugold ! 😄