* Add an "_" builtin to call into gettext
We already have gettext in C++ (if available), so it seems weird to
fork off a command to start it from script.
This is only for fish's own translations. There's no way to call into
other catalogs, it just translates all arguments separately.
This is faster by a factor of ~1000, which allows us to call
translations much more, especially from scripts.
E.g. making fish_greeting global by default would hurt cost-wise,
given that my fish starts up in 8ms and just calling the current `_`
function takes 2ms, and that would have two calls.
Incidentally, this also makes us rely on a weirdly defined function
less, so it:
Fixes#6804.
* docs: Add `_` docs
Let's see if that filename works out.
* Reword _ docs
