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 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