Modify the NS_ZERO state in the JSON number parser to allow
exponential notation with a zero coefficient (e.g. 0E-4).

The NS_ZERO state transition on 'E' was updated to move to the
NS_EX state rather than returning a syntax error. Similar change
was made for the NS_ZE1 (negative zero) starter state.

This allows accepted number grammar to include cases like:

- 0E4
- -0E-10

which were previously disallowed. Numeric parsing remains
the same for all other states.

Test cases are added to func_json.test to validate parsing for
various exponential numbers starting with zero coefficients.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services.