use editor.shell for formatter

[kirawi]

Objective

• Resolves expand tilde to home-directory for language formatter command args #5538

Solution

• Copied the way that the shell is used in helix_term::commands::shell_impl_async:

  helix/helix-term/src/commands.rs

  Lines 4996 to 5001 in b2e83f8

  	let mut process = Command::new(&shell[0]);
  	process
  	.args(&shell[1..])
  	.arg(cmd)
  	.stdout(Stdio::piped())
  	.stderr(Stdio::piped());

---

Changelog

now passes external formatters to editor.shell

[archseer]

We decided against this here #8064 (comment)