To prepare for an upcoming Utf16String, this migrates Utf16View to store
its data as a char16_t. Most function definitions are moved inline and
made constexpr.
This also adds a UDL to construct a Utf16View from a string literal:
auto string = u"hello"sv;
This let's us remove the NTTP Utf16View constructor, as we have found
that such constructors bloat binary size quite a bit.
When the cursor was positioned at the end of text,
attempting to move it left(using the left arrow key)
would fail because align_boundary() was rejecting
the end-of-text position as a valid boundary.
In the UTF-8 implementation, this prevents out-of-bounds access of the
underlying text data, as the ICU macro would essentially do something
akin to `text[text.length()]`.
The UTF-16 implementation already checks for out-of-bounds, but would
previously return 0. We now return an empty Optional in both impls. This
doesn't affect LibJS (the user of the UTF-16 impl), as it already does
bounds checking before invoking LibUnicode APIs.
