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/assistant/src/assistant_settings.rs

@@ -10,7 +10,7 @@ use serde::{
     de::{self, Visitor},
     Deserialize, Deserializer, Serialize, Serializer,
 };
-use settings::Settings;
+use settings::{Settings, SettingsSources};
 
 #[derive(Clone, Debug, Default, PartialEq)]
 pub enum ZedDotDevModel {
@@ -332,13 +332,12 @@ impl Settings for AssistantSettings {
     type FileContent = AssistantSettingsContent;
 
     fn load(
-        default_value: &Self::FileContent,
-        user_values: &[&Self::FileContent],
+        sources: SettingsSources<Self::FileContent>,
         _: &mut gpui::AppContext,
     ) -> anyhow::Result<Self> {
         let mut settings = AssistantSettings::default();
 
-        for value in [default_value].iter().chain(user_values) {
+        for value in sources.defaults_and_customizations() {
             let value = value.upgrade();
             merge(&mut settings.enabled, value.enabled);
             merge(&mut settings.button, value.button);