Indent guides (#11503)

Builds on top of existing work from #2249, but here's a showcase:


https://github.com/zed-industries/zed/assets/53836821/4b346965-6654-496c-b379-75425d9b493f

TODO:
- [x] handle line wrapping
- [x] implement handling in multibuffer (crashes currently)
- [x] add configuration option
- [x] new theme properties? What colors to use?
- [x] Possibly support indents with different colors or background
colors
- [x] investigate edge cases (e.g. indent guides and folds continue on
empty lines even if the next indent is different)
- [x] add more tests (also test `find_active_indent_index`)
- [x] docs (will do in a follow up PR)
- [x] benchmark performance impact

Release Notes:

- Added indent guides
([#5373](https://github.com/zed-industries/zed/issues/5373))

---------

Co-authored-by: Nate Butler <1714999+iamnbutler@users.noreply.github.com>
Co-authored-by: Remco <djsmits12@gmail.com>

crates/theme/src/settings.rs

@@ -325,6 +325,7 @@ impl ThemeSettings {
                 .status
                 .refine(&theme_overrides.status_colors_refinement());
             base_theme.styles.player.merge(&theme_overrides.players);
+            base_theme.styles.accents.merge(&theme_overrides.accents);
             base_theme.styles.syntax =
                 SyntaxTheme::merge(base_theme.styles.syntax, theme_overrides.syntax_overrides());