json: improve parsing and serializing of 'float' and 'double'

Up to now, the handling of type float was offloaded to the users of the
JSON utility, with the token JSON_TOK_FLOAT.

Improve handling of floating point types and support the types 'float'
and 'double' in a built-in way so that they can be directly parsed to
and serialized from variables (of type float or double).

The types are serialized in the shortest representation, either as a
decimal number or in scientific notation:
  * float (with JSON_TOK_FLOAT_FP): encoded with maximal 9 digits
  * double (with JSON_TOK_DOUBLE_FP): encoded with maximal 16 digits
  * NaN, Infinity, -Infinity: encoded and decoded as:
    {"nan_val":NaN,"inf_pos":Infinity,"inf_neg":-Infinity}

Enable the floating point functionality with the Kconfig option:
  JSON_LIBRARY_FP_SUPPORT=y

It requires a libc implementation with support for floating point
functions: strtof(), strtod(), isnan() and isinf().

Fixes: #59412
Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>