terminal: Improve default locale handling (#18967)

terminal: Improve default locale handling

* Use `LANG` instead of `LC_ALL` (`LC_ALL` is the highest priority which
will override any other end-user settings; when that isn't set things
fall back to separate `LC_*` variables; and when those aren't set things
fall back to `LANG`). [0]
* Only set `LANG` for our child if necessary (if it already exists in
the parent, then the child will inherit that, no need for us to do
anything)

[0]
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_02

Tested cases:

- `unset LANG ; cargo run`: locale inside zed's terminal is set to
`en_US.UTF-8`
- `export LANG=en_GB.UTF-8 ; cargo run`: locale inside zed's terminal is
set to `en_GB.UTF-8`

Release Notes:

- Use the system locale in the terminal instead of forcing `en_US.UTF-8`