Better JSON type mismatch errors

[RyanUnderhill]

This changes the JSON parsing to use a std::variant so there just a single OnValue handler vs OnString/OnNumber/OnBool/OnNull.

Previously a mismatched type would say

JSON Error: Unknown value: name at line 3 index 19

or it would say

JSON Error: Unknown value: name

if the name was known but the type of its value was wrong (example: #1146).

Now it'll give a much better error message, showing first the full path of the field being parsed, and then saying exactly how the types mismatch:

JSON Error: model:type - Expected a number but saw a string at line 3 index 19