Add Validate Syntax command - ⌃+⇧+V

Add a new command that does syntax validation on the JSON file and shows the
error message in a tooltip. Behaviour is consistent with other language
bundles where syntax validation is implemented.

The current reformat document shows syntax errors when the JSON is not valid but the user may not be interested in doing a document reformat, this adds a new command that is more consistent with the other bundles. The cursor is also moved to the error line.