Allow trailing commas in builtin JSONC schemas (#43854)

The JSON language server looks for a top-level `allowTrailingCommas`
flag to decide whether it should warn for trailing commas. Since the
JSONC parser for these builtin files can handles trailing commas, adding
this flag to the schema also prevents a warning for those commas.

I don't think there's an issue that is only for this specific issue, but
it relates to *many* existing / older issues:
- #18509
- #17487
- #40970
- #18509
- #21303

Release Notes:

- Suppress warning for trailing commas in builtin JSON files
(`settings.json`, `keymap.json`, etc.)

crates/settings/src/keymap_file.rs

@@ -15,6 +15,7 @@ use util::ResultExt as _;
 use util::{
     asset_str,
     markdown::{MarkdownEscaped, MarkdownInlineCode, MarkdownString},
+    schemars::AllowTrailingCommas,
 };
 
 use crate::SettingsAssets;
@@ -451,7 +452,9 @@ impl KeymapFile {
     /// Creates a JSON schema generator, suitable for generating json schemas
     /// for actions
     pub fn action_schema_generator() -> schemars::SchemaGenerator {
-        schemars::generate::SchemaSettings::draft2019_09().into_generator()
+        schemars::generate::SchemaSettings::draft2019_09()
+            .with_transform(AllowTrailingCommas)
+            .into_generator()
     }
 
     pub fn generate_json_schema_for_registered_actions(cx: &mut App) -> Value {

crates/settings/src/settings_store.rs

@@ -25,7 +25,7 @@ use std::{
 use util::{
     ResultExt as _,
     rel_path::RelPath,
-    schemars::{DefaultDenyUnknownFields, replace_subschema},
+    schemars::{AllowTrailingCommas, DefaultDenyUnknownFields, replace_subschema},
 };
 
 pub type EditorconfigProperties = ec4rs::Properties;
@@ -1010,6 +1010,7 @@ impl SettingsStore {
     pub fn json_schema(&self, params: &SettingsJsonSchemaParams) -> Value {
         let mut generator = schemars::generate::SchemaSettings::draft2019_09()
             .with_transform(DefaultDenyUnknownFields)
+            .with_transform(AllowTrailingCommas)
             .into_generator();
 
         UserSettingsContent::json_schema(&mut generator);