Support `editor::SelectAll` in Terminal (#2848)

![image](https://github.com/zed-industries/zed/assets/2690773/3aae1e6a-9993-4e65-8ed1-20f2f4b452df)

Allows to use `editor::SelectAll`(`cmd-a` by default) in Terminal to
select all text in it, for future copying.
Currently, does not try to be smart and trim the selected whitespaces
after the last prompt, and copies them too.

Release Notes:

- Support `editor::SelectAll` in Terminal