Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.
Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.
Implementation for new Variant types Callable, Signal, StringName.
Added support for PackedInt64Array and PackedFloat64Array.
Add generation of signal members as events, as well as support for
user created signals as events.
NOTE: As of now, raising such events will not emit the signal. As such,
one must use `EmitSignal` instead of raising the event directly.
Removed old ThreadLocal fallback class. It's safe to use thread_local now since
it's supported on all minimum versions of compilers we support.
