add .vyi to allowed Vyper extensions

[charles-cooper]

Description

vyper 0.4.0 adds .vyi files for defining standalone interfaces (see vyperlang/vyper@c6f457a / vyperlang/vyper#3663).

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.vyi
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/vyperlang/vyper/blob/master/vyper/builtins/interfaces/IERC20.vyi
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

potential distinguishing syntax (if necessary): def, event, struct and # for comments

[lildude]

I'm closing this PR as it is waaaay too early to submit it. There are only 17 files indexed on GitHub and they're owned by two users. This is a long long long way from meeting our requirements. Feel free to open a new PR once usage has met our requirements, and as a tip, when you do, include a sample file with the extension too 😉

[charles-cooper]

what do you mean, "include a sample file with the extension"? that's part of the PR description

[lildude]

We need the file added to the PR itself in the relevant directory under samples/. It's used to train the classifier which only reads the content of the samples/ directory. This is documented.