Add an application menu item for creating/opening a project-specific settings file (#2572)

Previously, project-specific settings were not discoverable. This PR
adds a `Zed > Preferences > Local Settings` application menu command
that creates a `.zed/settings.json` at the root of your current
worktree. This command works in both local and remote projects.

Limitations:

* Currently, if you have an empty project open, the command just shows a
notification that there are no folders open.
* The JSON-schema-based autocomplete is the same in local settings files
as in your main settings, even though not all settings can be locally
customized.

Release Notes:

- Added an application menu command - `Zed > Preferences > Local
Settings` for creating a folder-specific settings file.