Add `.c3i` extension to be recognized as the C3 language

Linguist already has C3 language. but it does not recognizes `.c3i` extension as a C3 file. `.c3i` are interface files used to declare the public API of a module without providing implementations.

Looking at the [PR](https://github.com/github-linguist/linguist/pull/7525) where C3 was added, the reason why .c3i extension was dropped is absence of .c3i example file?

If that was the case, c3 has an official vendor repository, we can took an example .c3i file from there. For example, [tigr.c3i](https://github.com/c3lang/vendor/blob/main/libraries/tigr.c3l/tigr.c3i)

Start by locating Linguist’s existing C3 language definition and the tests or sample-file conventions used for extension recognition. Use the referenced tigr.c3i file from the C3 vendor repository as the example, then verify that .c3i is recognized as C3 and that the relevant tests pass.

Written by the indexing model from the issue text.