Introduces a separate JSON schema for project settings that excludes
user-only settings like `auto_update`, `telemetry`, `vim_mode`, etc.
This provides more accurate autocomplete and validation when editing
`.zed/settings.json`.

- Add `SettingsStore::project_json_schema`
- Map `.zed/settings.json` to `zed://schemas/project_settings` schema URL

Release Notes:

- Improved autocomplete for the project settings file
(`.zed/settings.json`) to only include settings that are valid at the
project level, excluding user-only settings.