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>
