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).

• make use of Env in more places (all the shell interactions)
• 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]

Also, instead of string replacement, it may be more elegant to set the env on the command https://doc.rust-lang.org/std/process/struct.Command.html#method.env although I'm not sure if that might make configs non-cross-platform

[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 #3642