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/editor/src/display_map.rs

@@ -845,20 +845,7 @@ impl DisplaySnapshot {
             .buffer_line_for_row(buffer_row)
             .unwrap();
 
-        let mut indent_size = 0;
-        let mut is_blank = false;
-        for c in buffer.chars_at(Point::new(range.start.row, 0)) {
-            if c == ' ' || c == '\t' {
-                indent_size += 1;
-            } else {
-                if c == '\n' {
-                    is_blank = true;
-                }
-                break;
-            }
-        }
-
-        (indent_size, is_blank)
+        buffer.line_indent_for_row(range.start.row)
     }
 
     pub fn line_len(&self, row: DisplayRow) -> u32 {