Better JSON type mismatch errors (#1147)

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`