Additional Expansion Types and Variables for Command Expansions

[nouritsu]

The command expansion change made in #12527 has been an amazing change for people who love to integrate external tools into Helix.

However, I think command expansions could be 100x more powerful with just a few more additions.

Brainstorming,

• %reg{<reg>} would expand into the text stored in <reg>. This would allow users to setup clipboard mirroring on Windows + Linux VM users. For example, rebinding y to yank to system clipboard, but also pipe the contents of " to win32yank.
• %selected variable which would expand to the current selection. This would allow users to integrate common pipes like sort, bc or fmt into keybinds.
• %workspace_dir variable would expand into the directory that helix was opened in. This would allow users to open file explorers like yazi in more sensible starting directories.
• %focused_dir variable would expand into the directory that the current focused buffer is in, defaulting to %workspace_dir if in a scratch buffer. Another sensible starting directory for file explorers.
• %git_root variable would expand into the directory holding the current file's .git folder. Other variables related to version control would help speed up a lot of workflows.

Continue on!