Fix scrollbar marker settings (#10530)

Zed displays scrollbar markers of three types: git diffs, background
highlights and diagnostics. At the moment, the "background highlights"
markers are displayed for all the supported highlights:

- Occurences of the symbol under cursor.
- Search results.
- Scope boundaries (only works when a symbol is selected).
- Active hover popover position.

They all use the same color, which leads to confusing results. For
instance, in the following case I expect to see markers for the
`new_anchor` occurences in lines 43 and 47. But besides them, there're
also scope-markers for `ScrollAnchor` initialization block in lines 46
and 49, which makes me think that there're four places where
`new_anchor` appears.

<img width="740" alt="zed-scrollbar-markers"
src="https://github.com/zed-industries/zed/assets/2101250/78700e6b-fdd1-4c2f-beff-e564d8defc13">

Existing settings `selection` and `symbol_selection` in the `scrollbar`
section [don't work as
expected](https://github.com/zed-industries/zed/pull/10080#discussion_r1552325493),
which increases confusion.

This PR only leaves two types of bg-highlight-markers and provides
dedicated settings for them:

- Occurences of the symbol under cursor. Setting: `selected_symbol`,
default is `true`.
- Search results. Setting: `search_results`, default is `true`.

The `selection` and `symbol_selection` settings are not used anymore.

Release Notes:

- Breaking changes. Settings `selection` and `symbol_selection` in the
`scrollbar` section renamed to `search_results` and `selected_symbol`
respectively. Fixed the effect of these settings on which markers are
displayed on the scrollbar.

Optionally, include screenshots / media showcasing your addition that
can be included in the release notes.

- N/A

/cc @mrnugget

crates/editor/src/element.rs

@@ -965,11 +965,11 @@ impl EditorElement {
                 // Git
                 (is_singleton && scrollbar_settings.git_diff && snapshot.buffer_snapshot.has_git_diffs())
                     ||
-                    // Selections
-                    (is_singleton && scrollbar_settings.selections && editor.has_background_highlights::<BufferSearchHighlights>())
+                    // Buffer Search Results
+                    (is_singleton && scrollbar_settings.search_results && editor.has_background_highlights::<BufferSearchHighlights>())
                     ||
-                    // Symbols Selections
-                    (is_singleton && scrollbar_settings.symbols_selections && (editor.has_background_highlights::<DocumentHighlightRead>() || editor.has_background_highlights::<DocumentHighlightWrite>()))
+                    // Selected Symbol Occurrences
+                    (is_singleton && scrollbar_settings.selected_symbol && (editor.has_background_highlights::<DocumentHighlightRead>() || editor.has_background_highlights::<DocumentHighlightWrite>()))
                     ||
                     // Diagnostics
                     (is_singleton && scrollbar_settings.diagnostics && snapshot.buffer_snapshot.has_diagnostics())
@@ -2620,10 +2620,14 @@ impl EditorElement {
                             for (background_highlight_id, (_, background_ranges)) in
                                 background_highlights.iter()
                             {
-                                if (*background_highlight_id
-                                    == TypeId::of::<BufferSearchHighlights>()
-                                    && scrollbar_settings.selections)
-                                    || scrollbar_settings.symbols_selections
+                                let is_search_highlights = *background_highlight_id
+                                    == TypeId::of::<BufferSearchHighlights>();
+                                let is_symbol_occurrences = *background_highlight_id
+                                    == TypeId::of::<DocumentHighlightRead>()
+                                    || *background_highlight_id
+                                        == TypeId::of::<DocumentHighlightWrite>();
+                                if (is_search_highlights && scrollbar_settings.search_results)
+                                    || (is_symbol_occurrences && scrollbar_settings.selected_symbol)
                                 {
                                     let marker_row_ranges =
                                         background_ranges.into_iter().map(|range| {