Add `:theme-open` command to open theme TOML files.

[mathew-horner]

I don't think anything like this exists?

This is a nice to have if you like to hack on your own (or the pre-existing) themes. Rather than having to type :o ~/.config/helix/themes/... every time, you can just type :theme-open and you get all the same autocompletion that the :theme command gives you (minus the default themes as those don't have associated files).