fix: terminal freezing on `shell_insert_output`

This bug occurs on `shell_insert_output` and `shell_append_output`
commands.

The previous implementation would create a child process using the Rust
stdlib's `Command` builder. However, when nothing should be piped in
from the editor, the default value for `stdin` would be used. According
to the Rust stdlib documentation that is `Stdio::inherit` which will
make the child process inherit the parent process' stdin. This would
cause the terminal to freeze.

This change will set the child process' stdin to `Stdio::null` whenever
it doesn't pipe it. In the `if` statement where this change was made
there was an extra condition for windows that I am not sure if would
require some special treatment.

helix-term/src/commands.rs

@@ -4665,6 +4665,8 @@ fn shell_impl(
 
     if input.is_some() || cfg!(windows) {
         process.stdin(Stdio::piped());
+    } else {
+        process.stdin(Stdio::null());
     }
 
     let mut process = match process.spawn() {