Extend json path syntax to support field names containing reserved characters

[martint]

There's currently no way to extract values for fields in json structures whose names contain any of the special json path characters ($, [, ], ., etc.).

The syntax for json path expressions (for json_extract and friends) should be extended. One possible option is to allow bracket notation for field references (see http://goessner.net/articles/JsonPath/).

For example, to wanted to get the value for key "1.0" from the following structure:

{ "1.0": "apple", "2.0": "banana" }

The corresponding json path expression would be $["1.0"]. Note that the expression uses " instead of ' to make it fit more naturally within SQL strings.

[dain]

Fixed by #1466