PREUPLOAD: Enforce jsonlint in preupload

Checks syntax and simple brace balances for all JSON files.

Test: Edit any json file and try upload
Bug: 127794899
Change-Id: I59436ef5456f9826fe9d2dc76107d65194939418

PREUPLOAD.cfg

@@ -0,0 +1,2 @@
+[Builtin Hooks]
+jsonlint = true