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.

[baldwindavid]

@codingjerk Thanks for this; a real big win here is fixing the issue of the disappearing cursor over a ruler.

I read about some of the constraints and complications in #9256 and see that it's not completely straightforward how to approach.

Not a reviewer at all, but I do wonder if an option might be to introduce a couple settings:

Settings

| `ruler-style` | `bg` displays the ruler as a background color, while `char` displays the configured `ruler-char` | `bg` |
| `ruler-char`  | Specifies the character used to display the rulers when `ruler-style` is `char` | `|` |

The default style would be background so as not to break current settings. The default ruler character is set to |, but will only display if the ruler-style is set to char.

Theme Changes

Perhaps an additional theme key is then not necessary. Instead, the bg dictates the bg ruler-style just as it does now and fg dictates the char ruler-style.

"ui.virtual.ruler" = { bg = "cursorline", fg = "cursorline" }