Ability to use `$file` and `$pwd` in shell commands and formatter args

[AlexanderBrevig]

This is a work in progress, to satisfy my curiosity about a KISS way of going about it.

This will enable people to use $file and $pwd for insert-output and formatter command args (the latter is what prompted me to test this idea).

• [x] make use of Env in more places (all the shell interactions)
• [x] find issues and PRs to link to - I know there is pre-existing work on this
• [ ] figure out if the declarative approach is OK, or if we want something a bit more "sticky"

[AlexanderBrevig]

Alternative to #3393 , the other PR probably matches better but will be harder to reuse for things like LSP config?

[the-mikedavis]

I'm not sure how I feel about this vs #3393-style (kakoune-like) variables. The kakoune-like variables don't look like unix environment variables which is a double edged sword - nice for Windows users but maybe less intuitive for unix users

[kirawi]

Addresses https://github.com/helix-editor/helix/pull/3642