Default `#[schemars(deny_unknown_fields)] for json-language-server schemas (#33883)

Followup to #33678, doing the same thing for all JSON Schema files
provided to json-language-server

Release Notes:

* Added warnings for unknown fields when editing `tasks.json` /
`snippets.json`.

crates/util/src/util.rs

@@ -5,6 +5,7 @@ pub mod fs;
 pub mod markdown;
 pub mod paths;
 pub mod redact;
+pub mod schemars;
 pub mod serde;
 pub mod shell_env;
 pub mod size;