Support UTF-8 encoded surrogate pairs

[shooshx]

some systems emit utf8 strings with surrogate pairs encoded as two 3-byte sequences. uftcpp does not support such an encoding and throws an exception about an invalid code point when encountering it.

[nemtrif]

That is invalid UTF-8, so the library is doing the right thing.

[shooshx]

According to https://en.wikipedia.org/wiki/UTF-8#CESU-8
That is indeed invalid UTF-8 but there's a derivative standard callied CESU-8 in which it is infact valid.
There are systems (Android OS for instance) that do emit this encoding so the question of if its valid or not is irrelevant.
I would argue that "the right thing" would be to do the thing that is most useful and offers the broadest support. Otherwise any user who wants to use uftcpp on Android is going to run in to this issue and would eventually need to replace uftcpp with something else.