std.json: Fixed issue 12969. opIndexAssign operator for JSONValue added. Changed order of elements in enum JSON_TYPE so NULL is a default type when JSONValue is not initialized

Signed-off-by: Uranuz <neuranuz@gmail.com>
Deleted leading and trailing spaces inside *if* to match code style
Remove non `auto ref` opAssign overload

It conflicts with new one and should be completely covered by the nature
of auto ref.
Merge pull request #2607 from Dicebot/uranuz_issue_12969

Fix issue 12969 : opIndexAssign operator for JSONValue