feat: pre_keys/post_keys support for keystroke injection

[jxyyz]

Closes #17

What

Adds pre_keys and post_keys fields to SendItem and action-level config. This allows sending keystrokes before/after pasting text to tmux targets - useful for TUI apps that need state preparation (e.g. i for vim insert mode, C-c to cancel current input).

Usage example

wiremux.send({
  value = "explain this code",
  pre_keys = "i",
  submit = true,
})

-- action-level defaults
wiremux.send("hello", {
  pre_keys = { "C-c" },
  post_keys = { "Escape" },
})

Status

This is a first working prototype - pushed so it's available for testing and review. I'll be validating this further on my side.

Open to feedback and suggestions on the approach.

[jxyyz]

There is probably the same problem that was resolved in #15.

It appears when I try to replace submit = true option with post_keys = "Enter"

I'll check this later and adjust my implementation to the solution provided in that issue.