feat: add workspace_directory and current_working_directory variable expansions

[nik-rev]

Use case: Recently Yazi added out-of-the-box support for usage inside of Helix with with command:

[keys.normal]
space.E = [
  ':sh rm -f /tmp/unique-file',
  ':insert-output yazi %{buffer_name} --chooser-file=/tmp/unique-file',
  ':insert-output echo "\x1b[?1049h" > /dev/tty',
  ':open %sh{cat /tmp/unique-file}',
  ':redraw',
]

This replaces file_explorer_in_current_buffer_directory for me with a Yazi picker

I would like to be able to replace these two commands with Yazi as well:

• file_explorer (uses workspace root)
• file_explorer_in_current_directory (cwd)

This PR allows for that. Now you can do this too:

# file_explorer
space.e = [
  ':sh rm -f /tmp/unique-file2',
  ':insert-output yazi %{workspace_directory} --chooser-file=/tmp/unique-file2',
  ':insert-output echo "\x1b[?1049h" > /dev/tty',
  ':open %sh{cat /tmp/unique-file2}',
  ':redraw',
]
# file_explorer_in_current_working_dir
space.X = [
  ':sh rm -f /tmp/unique-file3',
  ':insert-output yazi %{workspace_directory} --chooser-file=/tmp/unique-file3',
  ':insert-output echo "\x1b[?1049h" > /dev/tty',
  ':open %sh{cat /tmp/unique-file3}',
  ':redraw',
]