Avoid using shell commands

[kdkasad]

Instead of using shell commands via io.popen() and os.execute(), this PR changes the code to open a handle to the standard output TTY, and write to it directly.

This improves plugin load time from ~8 ms to ~1 ms (on my laptop), which is nice. It also makes execution slightly faster each time the background is updated, which is not noticeable on fast machines, but is on slower ones. (I see a noticeable lag in background color updates on a slow university server without this change.)

This PR also replaces the deprecated vim.api.nvim_get_hl_by_name() function with vim.api.nvim_get_hl().

I've tested this in Ghostty and in a tmux session, and both work as expected. I do not have Kitty installed, so I haven't tested the Kitty-specific functionality.

[PowerUser64]

You can remove the last printf call if you replace line 33 with this: tty:write '\x1b]30001\x07'. Also, I tested these changes with kitty and it all seems to work just fine!

[kdkasad]

You can remove the last printf call if you replace line 33 with this: tty:write '\x1b]30001\x07'. Also, I tested these changes with kitty and it all seems to work just fine!

Thanks! Added that change.