feat: Use custom character to draw editor rulers

[codingjerk]

This PR allows selecting custom character to draw editor rulers and it contains fixes and improvements on PR #9256

Differences from previous PR

1. No unnecessary formatting changes
2. No unnecessary string allocation for every character drawn
3. Separate theme key (ui.virtual.ruler.char) if editor.ruler-char is set to allow using different styles for "background ruler" and "character ruler"
4. Ruler character is drawn under the text

Screenshot

Default theme with config

[editor]
  rulers = [ 40, 60 ]
  ruler-char = "¦"

[codingjerk]

I believe I also need to add theme key ui.virtual.ruler.char to every theme in helix.

I believe reasonable value for it is the same as ui.virtual.indent-guide. Using bg from ui.virtual.ruler as fg doesn't work well — in some themes it's pretty hard to see "character rulers" since they're thinner.

I'll wait for reviewers to approve this approach, before adding new key to every theme.