Add lingua franca language

[oowekyala]

This adds support for the Lingua Franca language (LF). LF is a coordination language that embeds fragments of another language (eg Rust, C, Python). For that reason tokei's embedded language feature is very valuable. Here's a sample output when run on this repo:

 Language            Files        Lines         Code     Comments       Blanks
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
...
 Lingua Franca          87         8964         4379         2567         2018
 |- C                   30         2451         1923          307          221
 |- C++                 24         2120         1659          238          223
 |- Python               1           14           14            0            0
 |- Rust                21         1967         1670          141          156
 |- TypeScript          10          429          384           23           22
 (Total)                          15945        10029         3276         2640

[XAMPPRocky]

I don't think we should have a warn! here, because people use it as a library, and they may not want to have it warn when getting None can be expected behaviour.

[oowekyala]

Good point, thanks! I'll change that

[oowekyala]

Actually all other such methods like from_file_extension use a warn! call, so maybe this change should be part of another PR?

[XAMPPRocky]

Thank you for your PR! I'll try to review this more throughly in the future 🙂

[XAMPPRocky]

Thank you for your PR, and congrats on your first contribution! 🎉