json: Support floating point values NaN and Infinity for numbers

Add support to decode the special floating point values NaN, Infinity
and -Infinity. For example:
  {"nan_val":NaN,"inf_pos":Infinity,"inf_neg":-Infinity}

Note that this commit is a preparation for the built-in support of
floating point values and these are only accepted when compiled with
the flag -DCONFIG_JSON_LIBRARY_FP_SUPPORT.

Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>