parse_constant receives '-NaN', breaking json-module-style hooks #152

A parse_constant hook written to the json module's contract breaks on -NaN, because pyjson5 passes the signed spelling to the hook instead of normalizing it:

import json5

HOOK = {'NaN': 'nan', 'Infinity': 'inf', '-Infinity': '-inf'}.__getitem__

json5.loads('NaN', parse_constant=HOOK)         # ok
json5.loads('-Infinity', parse_constant=HOOK)   # ok
json5.loads('-NaN', parse_constant=HOOK)        # KeyError: '-NaN'

json's documented contract only ever passes 'NaN', 'Infinity', or '-Infinity' to parse_constant, so code written against it assumes a closed set. -NaN is legal JSON5, so this is reachable with perfectly ordinary input.

The inconsistency is especially visible next to the + spelling, which already arrives normalized (the parser drops the plus sign):

json5.loads('+NaN', parse_constant=HOOK)  # ok — hook receives 'NaN'

Two possible resolutions:

• normalize like the default parser does (_fp_constant_parser already handles -NaN via float('-nan')), so hooks receive 'NaN' for both NaN and -NaN, or
• keep passing the literal but document it next to the existing parse_int caveat in the load/loads/parse docstrings (the docstrings already warn that parse_int sees 0x… spellings).

The first keeps drop-in compatibility with json-shaped hooks, which I suspect is worth more given the library already mirrors json's API so closely.