Declare explicit specializations for CharStringT

[mihe]

Related to #1150.

I'm in the process of upgrading godot-cpp from 1009da4 to 3b3f357, and when compiling with clang-cl (and maybe regular Clang as well?) I seem to end up with the following compilation error:

src/variant/char_string.cpp(95,32): error: explicit specialization of 'get_data' after instantiation
const char *CharStringT<char>::get_data() const {
                               ^
include/godot_cpp/core/type_info.hpp(354,1): note: implicit instantiation first required here
MAKE_TYPED_ARRAY_INFO(bool, Variant::BOOL)
^
include/godot_cpp/core/type_info.hpp(342,128): note: expanded from macro 'MAKE_TYPED_ARRAY_INFO'
                        return make_property_info(Variant::Type::ARRAY, "", PROPERTY_HINT_ARRAY_TYPE, Variant::get_type_name(m_variant_type).utf8().get_data()); \
                                                                                                                                                    ^

From what I can tell it seems like it gets cranky over the fact that the specializations made for CharStringT::get_data() weren't declared first. Searching around a bit seems to support that this should be required, although the example shown there is a bit different.

Weirdly enough it compiles just fine with latest MSVC. I haven't tried GCC, but I assume CI would have caught that already.

Anyway, this PR fixes this by simply declaring the specializations.

[dsnopek]

Thanks! This makes sense to me

[dsnopek]

Cherry-picked for 4.1 in PR #1281