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>

assets/settings/default.json

@@ -556,18 +556,10 @@
     "C": {
       "format_on_save": "off"
     },
-    "Plain Text": {
-      "soft_wrap": "preferred_line_length"
-    },
-    "Elixir": {
-      "tab_size": 2
-    },
     "Gleam": {
       "tab_size": 2
     },
     "Go": {
-      "tab_size": 4,
-      "hard_tabs": true,
       "code_actions_on_format": {
         "source.organizeImports": true
       }
@@ -575,34 +567,6 @@
     "Make": {
       "hard_tabs": true
     },
-    "Markdown": {
-      "tab_size": 2,
-      "soft_wrap": "preferred_line_length"
-    },
-    "JavaScript": {
-      "tab_size": 2
-    },
-    "Terraform": {
-      "tab_size": 2
-    },
-    "TypeScript": {
-      "tab_size": 2
-    },
-    "TSX": {
-      "tab_size": 2
-    },
-    "YAML": {
-      "tab_size": 2
-    },
-    "JSON": {
-      "tab_size": 2
-    },
-    "OCaml": {
-      "tab_size": 2
-    },
-    "OCaml Interface": {
-      "tab_size": 2
-    },
     "Prisma": {
       "tab_size": 2
     }