allow looking up language by filename

[tmm1]

fixes #21

cc @paul-gauthier @simonw

[grantjenks]

Should this go in py-tree-sitter instead? Seems like a simple script to generate a language file which would be generally useful. I understand it’s related here to the binaries included in the package but I’d rather not maintain it myself.

[tmm1]

Yea that's a fair point. Perhaps py-tree-sitter should generate a languages_index.py alongside the languages.so, and then all you have to do is copy it over.

I'll work on an upstream PR, but if it gets stuck in limbo perhaps we can ship this and refactor later.