Editor tab bar settings (#7356)

This PR is another step to tabless editing (#6424, #4963). It adds
support for tab bar settings that allow the user to change its placement
or to hide completely.

Configuraton:

```json
"tab_bar": {
  "show": true
}
```

Placemnet options are "top", "bottom" and "no".

This PR intentionally doesn't affect tab bars of other panes (Terminal
for instance) to keep code changes small. I guess we'll do the rest in
separate PRs.

Release Notes:

- Added support for configuring the editor tab bar (part of #6424,
#4963).

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/workspace/src/workspace_settings.rs

@@ -60,11 +60,16 @@ pub struct WorkspaceSettingsContent {
 
 #[derive(Deserialize)]
 pub struct TabBarSettings {
+    pub show: bool,
     pub show_nav_history_buttons: bool,
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
 pub struct TabBarSettingsContent {
+    /// Whether or not to show the tab bar in the editor.
+    ///
+    /// Default: top
+    pub show: Option<bool>,
     /// Whether or not to show the navigation history buttons in the tab bar.
     ///
     /// Default: true