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>

assets/settings/default.json

@@ -316,6 +316,8 @@
   "autosave": "off",
   // Settings related to the editor's tab bar.
   "tab_bar": {
+    // Whether or not to show the tab bar in the editor
+    "show": true,
     // Whether or not to show the navigation history buttons.
     "show_nav_history_buttons": true
   },