Move and rename messages template file

There is no reason to have this file clutter the repo root.
Move it into the `po` directory, and give it a more descriptive name.

Inspired by this discussion:
https://github.com/fish-shell/fish-shell/pull/11463#discussion_r2083453275

I use `template.po` instead of `message.po-template` to be more compatible with
automatic filetype detection. (e.g. vim/nvim detect the former as a po file, but
the latter as a conf file)

build_tools/update_translations.fish

@@ -23,7 +23,7 @@
 set -gx LC_ALL C.UTF-8
 
 set -l build_tools (status dirname)
-set -l template_file $build_tools/../messages.pot
+set -l template_file $build_tools/../po/template.po
 set -l po_dir $build_tools/../po
 
 set -l extract