pg_overexplain produces invalid JSON with RANGE_TABLE option

Hi Hackers,

It appears that pg_overexplain produces invalid JSON output when
EXPLAIN (FORMAT JSON, RANGE_TABLE) is used. The "Unprunable RTIs" and
"Result RTIs" properties are emitted as key:value pairs directly inside
the "Range Table" JSON array, which is structurally invalid.

Attempted to fix this by moving ExplainCloseGroup() before the two
overexplain_bitmapset()
calls, so the properties are emitted as siblings of "Range Table" in the
parent object rather
than inside the array.

Attached a patch to address this which also includes a test.

Repro:

LOAD 'pg_overexplain';
  CREATE TABLE t1 (id serial PRIMARY KEY, val text);
  CREATE TABLE t2 (id serial PRIMARY KEY, a_id int REFERENCES t1(id), data
text);
  INSERT INTO t1 VALUES (1, 'x'), (2, 'y');
  INSERT INTO t2 VALUES (1, 1, 'd1'), (2, 2, 'd2');

EXPLAIN (FORMAT JSON, RANGE_TABLE)
   EXPLAIN (FORMAT JSON, RANGE_TABLE)
    SELECT * FROM t1 a JOIN t2 b ON a.id = b.a_id WHERE a.id = 1; "
| tail -n +2 | python3 -c "import json,sys; json.loads(sys.stdin.read());
print('VALID JSON')"

Thanks,
Satya