Respect line number formatting

[mvllow]

Bold (and possibly italic or other styles) are removed when the line number highlight changes. The correct format is restored once modes is no longer changing the line number highlight.

Can be reproduced via:

:set termguicolors
:colorscheme lunaperche
:set number
:set cursorline

The active line number should be bold by default. Once using a modes modifier, e.g. y, the entire line changes to yellow and the line number is no longer bold.

[fitrh]

I think this also requires refactoring all :highlight instances with nvim_set_hl. I've always used neovim nightly so I don't remember exactly when nvim_set_hl and nvim_get_hl were introduced. If we decided to refactor, I think we also need to mention the minimum required neovim version

[mvllow]

Looks like those were added in 0.9.0 via neovim/neovim#22693. This issue isn't that important to me personally as I usually don't have line numbers on. Maybe I can revisit this in the future unless someone wants to take it on :)