Linguist: Support TexInfo

[FranklinYu]

Example file:

• https://github.com/ruby-debug/ruby-debug-ide/blob/master/doc/protocol-spec.texi
• https://github.com/rmloveland/scsh-manual-texinfo/blob/master/scsh.texi

Official website for GNU Texinfo: https://www.gnu.org/software/texinfo/

[Alhadis]

We can register it as a language on GitHub, but there won't be syntax highlighting as every TexInfo grammar out there is GPL-licensed 👎 (I've searched before).

I've been tempted to write one in the past, but I'm not familiar with the format, sadly.

[stale[bot]]

This issue has been automatically marked as stale because it has not had activity in a long time. If this issue is still relevant and should remain open, please reply with a short explanation (e.g. "I have checked the code and this issue is still relevant because ___."). Thank you for your contributions.

[jchook]

Another example: https://github.com/westes/flex/blob/master/doc/flex.texi

[Alhadis]

I've written an original (non-copylefted) grammar that will fix this. See #4589.