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>

assets/themes/gruvbox/gruvbox.json

@@ -5,6 +5,15 @@
     {
       "name": "Gruvbox Dark",
       "appearance": "dark",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#5b534dff",
         "border.variant": "#494340ff",
@@ -379,6 +388,15 @@
     {
       "name": "Gruvbox Dark Hard",
       "appearance": "dark",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#5b534dff",
         "border.variant": "#494340ff",
@@ -753,6 +771,15 @@
     {
       "name": "Gruvbox Dark Soft",
       "appearance": "dark",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#5b534dff",
         "border.variant": "#494340ff",
@@ -1127,6 +1154,15 @@
     {
       "name": "Gruvbox Light",
       "appearance": "light",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#c8b899ff",
         "border.variant": "#ddcca7ff",
@@ -1501,6 +1537,15 @@
     {
       "name": "Gruvbox Light Hard",
       "appearance": "light",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#c8b899ff",
         "border.variant": "#ddcca7ff",
@@ -1875,6 +1920,15 @@
     {
       "name": "Gruvbox Light Soft",
       "appearance": "light",
+      "accents": [
+        "#cc241dff",
+        "#98971aff",
+        "#d79921ff",
+        "#458588ff",
+        "#b16286ff",
+        "#689d6aff",
+        "#d65d0eff"
+      ],
       "style": {
         "border": "#c8b899ff",
         "border.variant": "#ddcca7ff",