This PR allows you to customize Zed's settings within a particular
folder by creating a `.zed/settings.json` file within that folder.

Todo

* [x] respect folder-specific settings for local projects
* [x] respect folder-specific settings in remote projects
* [x] pass a path when retrieving editor/language settings
* [x] pass a path when retrieving copilot settings
* [ ] update the `Setting` trait to make it clear which types of
settings are locally overridable

Release Notes:

* Added support for folder-specific settings. You can customize Zed's
settings within a particular folder by creating a `.zed` directory and a
`.zed/settings.json` file within that folder.