Improve support for variable bullet lengths

[f4z3r]

I personally like to still have the number of bullets equal to the heading level to simpler overview on what level I am at. Before this PR, it was already possible to set more than one character for a bullet, but the virtual text was then misaligned. This PR allows to define several characters for the bullets ensuring they are correctly aligned as long as the character count is less or equal to the heading level on which they will be applied. Thus, configurations such as the following become possible and are properly aligned: https://github.com/f4z3r/nix/blob/3dfe0f2a28920f1598662c948b846651f551c4f3/home/apps/nvim/plugin/headlines.lua#L4

Existing configurations are not affected by the change.

[lukas-reineke]

Thanks, but string.len returns the byte length. This doesn't work correctly for UTF-8. Please use strwidth instead

[f4z3r]

@lukas-reineke ah good point! I updated the PR.

[lukas-reineke]

thanks