The Windows and GLFW embeddings both had cases of missing returns after
calling `Error` on a method channel result object, leading to incorrect
double-calls on the result if the error paths were ever hit.
Instead of a hand-rolled discriminated union (originally used to avoid a C++17
dependency, which is no longer an issue), implement EncodableValue as a
std::variant. Rather than simply changing the internals, this makes EncodableValue
a minimal std::variant subclass with only a handful of added methodS, replacing
the old IsFoo/FooValue APIs with the standard std::holds_alternative/std::get,
so that plugin code will use a standard-based API rather than a Flutter-specific
API for wrapped values.
This is a breaking change for Windows and GLFW plugins. In the short
term USE_LEGACY_ENCODABLE_VALUE can be set in builds to use the old
version, to separate rolling from updating.
Fixes https://github.com/flutter/flutter/issues/61970
