Expanding envs in language config

[kj-crypto]

This is simple expansion of the shell variables in languages.toml config. This allows to write such expression

[[language]]
name = "go"
formatter = { command = "${HOME}/go/bin/goimports" }

The envs to expand are limited by AllowedEnv enum and currently only allowed on linux OS.
Tested with hx --health go and during working on golang source code

[nik-rev]

Helix already has a system for expansions, e.g. :echo %{cursor_line}

It will be easier and more consistent to re-use that rather than Regex

[kj-crypto]

Thanks for pointing it out. I will integrate it with helix::stdx