Update tabline truncation logic, cache tabline for performance

[iofq]

No description provided.

[iofq]

There is still a bug in here somewhere:

files open:

Jump to buffer:
  z → thisiisanothertrlaknfklajsndfk.lua
  x → completion.lua
  c → flake.nix
  a → yeahthisfilenameisreallylongsowhatareyougonnado.php
  s → .stylua.toml

vim.o.columns = 124 columns

behavior

Tabline truncation is being incorrectly calculated in the first shot, as in the second shot it fits fine in the screen.

[iofq]

running :wa does not detect now-unmodified files correctly (doesn't refresh)