Warn about unknown fields when editing settings json (#33678)

Closes #30017

* While generating the settings JSON schema, defaults all schema
definitions to reject unknown fields via `additionalProperties: false`.

* Uses `unevaluatedProperties: false` at the top level to check fields
that remain after the settings field names + release stage override
field names.

* Changes json schema version from `draft07` to `draft_2019_09` to have
support for `unevaluatedProperties`.

Release Notes:

- Added warnings for unknown fields when editing `settings.json`.

crates/snippet_provider/src/format.rs

@@ -12,7 +12,7 @@ pub struct VsSnippetsFile {
 
 impl VsSnippetsFile {
     pub fn generate_json_schema() -> Value {
-        let schema = schemars::generate::SchemaSettings::draft07()
+        let schema = schemars::generate::SchemaSettings::draft2019_09()
             .into_generator()
             .root_schema_for::<Self>();