Detect Vim config files as 'vim' file type

[rhysd]

.vimrc, vimrc, _vimrc, .gvimrc, gvimrc, _gvimrc are file names for Vim configurations and written in Vim script.

This PR adds them for vim file type detection.

Documents:

• vimrc config: https://github.com/vim/vim/blob/2446ec9b567ce2b72bd06d121f200f40bbdc8a84/runtime/doc/starting.txt#L789-L795
• gvimrc config: https://github.com/vim/vim/blob/2446ec9b567ce2b72bd06d121f200f40bbdc8a84/runtime/doc/gui.txt#L97-L102

[BurntSushi]

Thanks! LGTM, but could you please ensure your changes are formatted in a way that is consistent with the surrounding code? In particular, please wrap your code to 79 columns (inclusive).

[rhysd]

My text editor automatically applies rustfmt, but let me ensure it is formatted.

[rhysd]

Ah, ok, it is marked as #[rustfmt::skip]. Let me fix it.

[rhysd]

@BurntSushi I fixed all lines which excludes exceed the limit at 39eabf0.

[BurntSushi]

Thanks!