`script_extensions` should return an `optional<span<Script>>`

[christianparpart]

script_extensions should return an optional<span<Script>>

this requires C++20 though. maybe auto-detect if that is the case, and if not, provide a custom span<T> type for that.

also make use of that in contour when implemented.

So the signature for the additional function would be:

std::optional<std::span<Script>> script_extensions(char32_t _codepoint) noexcept;