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/editor_settings.rs

@@ -58,8 +58,8 @@ pub struct Toolbar {
 pub struct Scrollbar {
     pub show: ShowScrollbar,
     pub git_diff: bool,
-    pub selections: bool,
-    pub symbols_selections: bool,
+    pub selected_symbol: bool,
+    pub search_results: bool,
     pub diagnostics: bool,
 }
 
@@ -194,14 +194,14 @@ pub struct ScrollbarContent {
     ///
     /// Default: true
     pub git_diff: Option<bool>,
-    /// Whether to show buffer search result markers in the scrollbar.
+    /// Whether to show buffer search result indicators in the scrollbar.
     ///
     /// Default: true
-    pub selections: Option<bool>,
-    /// Whether to show symbols highlighted markers in the scrollbar.
+    pub search_results: Option<bool>,
+    /// Whether to show selected symbol occurrences in the scrollbar.
     ///
     /// Default: true
-    pub symbols_selections: Option<bool>,
+    pub selected_symbol: Option<bool>,
     /// Whether to show diagnostic indicators in the scrollbar.
     ///
     /// Default: true