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

assets/settings/default.json

@@ -145,10 +145,10 @@
     "show": "auto",
     // Whether to show git diff indicators in the scrollbar.
     "git_diff": true,
-    // Whether to show selections in the scrollbar.
-    "selections": true,
-    // Whether to show symbols selections in the scrollbar.
-    "symbols_selections": true,
+    // Whether to show buffer search results in the scrollbar.
+    "search_results": true,
+    // Whether to show selected symbol occurrences in the scrollbar.
+    "selected_symbol": true,
     // Whether to show diagnostic indicators in the scrollbar.
     "diagnostics": true
   },