Add the ability for extensions to provide language settings (#10296)

This PR adds the ability for extensions to provide certain language
settings via the language `config.toml`.

These settings are then merged in with the rest of the settings when the
language is loaded from the extension.

The language settings that are available are:

- `tab_size`
- `hard_tabs`
- `soft_wrap`

Additionally, for bundled languages we moved these settings out of the
`settings/default.json` and into their respective `config.toml`s .

For languages currently provided by extensions, we are leaving the
values in the `settings/default.json` temporarily until all released
versions of Zed are able to load these settings from the extension.

---

Along the way we ended up refactoring the `Settings::load` method
slightly, introducing a new `SettingsSources` struct to better convey
where the settings are being loaded from.

This makes it easier to load settings from specific locations/sets of
locations in an explicit way.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/theme/src/settings.rs

@@ -14,7 +14,7 @@ use schemars::{
 };
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
-use settings::{Settings, SettingsJsonSchemaParams};
+use settings::{Settings, SettingsJsonSchemaParams, SettingsSources};
 use std::sync::Arc;
 use util::ResultExt as _;
 
@@ -316,14 +316,11 @@ impl settings::Settings for ThemeSettings {
 
     type FileContent = ThemeSettingsContent;
 
-    fn load(
-        defaults: &Self::FileContent,
-        user_values: &[&Self::FileContent],
-        cx: &mut AppContext,
-    ) -> Result<Self> {
+    fn load(sources: SettingsSources<Self::FileContent>, cx: &mut AppContext) -> Result<Self> {
         let themes = ThemeRegistry::default_global(cx);
         let system_appearance = SystemAppearance::default_global(cx);
 
+        let defaults = sources.default;
         let mut this = Self {
             ui_font_size: defaults.ui_font_size.unwrap().into(),
             ui_font: Font {
@@ -348,15 +345,15 @@ impl settings::Settings for ThemeSettings {
             theme_overrides: None,
         };
 
-        for value in user_values.iter().copied().cloned() {
-            if let Some(value) = value.buffer_font_family {
+        for value in sources.user.into_iter().chain(sources.release_channel) {
+            if let Some(value) = value.buffer_font_family.clone() {
                 this.buffer_font.family = value.into();
             }
             if let Some(value) = value.buffer_font_features {
                 this.buffer_font.features = value;
             }
 
-            if let Some(value) = value.ui_font_family {
+            if let Some(value) = value.ui_font_family.clone() {
                 this.ui_font.family = value.into();
             }
             if let Some(value) = value.ui_font_features {
@@ -373,7 +370,7 @@ impl settings::Settings for ThemeSettings {
                 }
             }
 
-            this.theme_overrides = value.theme_overrides;
+            this.theme_overrides = value.theme_overrides.clone();
             this.apply_theme_overrides();
 
             merge(&mut this.ui_font_size, value.ui_font_size.map(Into::into));