feat(core): defer clipboard because xsel and pbcopy can be slow

[abeldekat]

📑 Description

See this discussion in LazyVim
Also see this pr in LazyVim

TLDR:
Startup time performance is affected quite significantly when the clipboard provider is xsel(linux) or pbcopy(macos). I expect an improvement in these cases, especially on older pc's.

This PR makes sure that vim.opt.clipboard is only applied after UiEnter

[mehalter]

Thanks so much for this recommendation! I modified the code slightly to decrease the number of conditional checks and merged it manually with this commit: 3bc1868

I made sure to set the authorship as well :) Thanks again!

[abeldekat]

Thanks! Happy to contribute!