json schema: set schema version to draft-4, clarify taplo support

"$comment" is not officially supported by draft-4, but it is in newer
drafts, and JSON schema readers are supposed to skip fields that
are not known to them.

cli/src/config-schema.json

@@ -1,5 +1,6 @@
 {
-    "$schema": "http://json-schema.org/draft-07/schema",
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "$comment": "`taplo` and the corresponding VS Code plugins only support draft-04 verstion of JSON Schema, see <https://taplo.tamasfe.dev/configuration/developing-schemas.html>. draft-07 is mostly compatible with it, newer versions may not be.",
     "title": "Jujutsu config",
     "type": "object",
     "description": "User configuration for Jujutsu VCS. See https://jj-vcs.github.io/jj/latest/config/ for details",

cli/tests/test_util_command.rs

@@ -23,9 +23,10 @@ fn test_util_config_schema() {
     let stdout = test_env.jj_cmd_success(test_env.env_root(), &["util", "config-schema"]);
     // Validate partial snapshot, redacting any lines nested 2+ indent levels.
     insta::with_settings!({filters => vec![(r"(?m)(^        .*$\r?\n)+", "        [...]\n")]}, {
-        assert_snapshot!(stdout, @r###"
+        assert_snapshot!(stdout, @r#"
         {
-            "$schema": "http://json-schema.org/draft-07/schema",
+            "$schema": "http://json-schema.org/draft-04/schema",
+            "$comment": "`taplo` and the corresponding VS Code plugins only support draft-04 verstion of JSON Schema, see <https://taplo.tamasfe.dev/configuration/developing-schemas.html>. draft-07 is mostly compatible with it, newer versions may not be.",
             "title": "Jujutsu config",
             "type": "object",
             "description": "User configuration for Jujutsu VCS. See https://jj-vcs.github.io/jj/latest/config/ for details",
@@ -35,7 +36,7 @@ fn test_util_config_schema() {
                 [...]
             }
         }
-        "###);
+        "#);
     });
 }