Support DECSLRM (SMGLR, SMGLP, SMGRP) - Set Left and Right Margins for better scrolling performance in multi-pane TUIs

[aktau]

Ref. #2358

Also known as Set Left and Right Margins. This feature is used by terminal applications like tmux and (neo)vim to optimize pane scrolling. Real xterm supports it, many not-real-xterms don't, so Neovim patches out this functionality for xterm pretenders (which it tries to detect via the XTERM_VERSION environment variable).

Right now, forcing Neovim to use the smglr results in this: https://asciinema.org/a/Qcgm1znQy3nE8DnCc0jnrjXMi (versus not using them).

[jerch]

@aktau Yepp, left-right scroll margin support is quite important for any more modern TUI-like terminal interaction with partial screen updates. I wanted to implement this for quite some time, but kinda never got down to it.

If anyone feels ready to implement this in the near future - plz go for it. I'll mark the issue as "help wanted", but plz note that it involves quite some changes in several sequence functions and needs some knowledge of terminal mechs, so it is not a "good first issue".

If no one picks it up - I'll see if and when I can get down it myself, as it is still on my long-term todo list.

[aktau]

Thanks! Looking forward to anyone having a shot at this.

[jerch]

Did I really write modern? 😆