lib/json/json: Add function to unescape JSON-encoded strings

lib/json/json: Add function to unescape JSON-encoded strings

JSON strings require certain characters to be encoded, either by using
a single reverse solidus character "\" for a set of popular characters,
or by using a Unicode representation of "\uXXXXX". The jsmn library
doesn't handle unescaping for us, so we must implement this functionality
for ourselves.

Add a new function grub_json_unescape() that takes a potentially
escaped JSON string as input and returns a new unescaped string.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <dkiper@net-space.pl>