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/task/src/debug_format.rs

@@ -357,6 +357,7 @@ impl DebugTaskFile {
             "$schema": meta_schema,
             "title": "Debug Configurations",
             "description": "Configuration for debug scenarios",
+            "allowTrailingCommas": true,
             "type": "array",
             "items": {
                 "type": "object",

crates/task/src/task_template.rs

@@ -4,7 +4,7 @@ use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
 use std::path::PathBuf;
-use util::schemars::DefaultDenyUnknownFields;
+use util::schemars::{AllowTrailingCommas, DefaultDenyUnknownFields};
 use util::serde::default_true;
 use util::{ResultExt, truncate_and_remove_front};
 
@@ -118,6 +118,7 @@ impl TaskTemplates {
     pub fn generate_json_schema() -> serde_json::Value {
         let schema = schemars::generate::SchemaSettings::draft2019_09()
             .with_transform(DefaultDenyUnknownFields)
+            .with_transform(AllowTrailingCommas)
             .into_generator()
             .root_schema_for::<Self>();