[patch][bug #56774] support Rust programming language

[Kévin]

I made two patches to add Rust programming language support. One is for 
the Rust format strings and the other is for Rust support in xgettext.

They were too big to be accepted in this mailing list so they're 
available at: <https://gitlab.gnome.org/-/snippets/2934>.

Besides the tests in the patches, I tested it on a Rust project to make 
sure it works.

I didn't write any test or example that requires it to use gettext at 
runtime because it requires the cargo package manager and I'm not sure 
how to mix this with a Makefile.