Add VARIANT and PROPVARIANT support

[kennykerr]

This update adds first-class support for VARIANT and PROPVARIANT to the windows-core crate and by extension the windows crate. This does not affect the windows-sys crate.

Creating a variant is now very simple:

let v = VARIANT::from(345u16);

The TryFrom trait can be used to query for an expected type:

assert_eq!(u16::try_from(&v)?, 345u16);

Most primitive types, including BSTR, are supported:

let v = VARIANT::from(BSTR::from("hello"));
assert_eq!(BSTR::try_from(&v)?, "hello");

Natural type coercion is also support:

let v = VARIANT::from(3.5f64);
assert_eq!(BSTR::try_from(&v)?, "3.5");

Standard traits like Drop, Clone, PartialEq, and Debug are provided:

assert_eq!(format!("{v:?}"), "VARIANT { type: 5, value: 3.5 }");

Fixes: #539

[mwcampbell]

For what it's worth, if this is released in its current state, AccessKit won't be able to upgrade to this version, unless there's some escape hatch I can still use to create a VARIANT from an IUnknown and return that VARIANT in an implemented interface method.

[kennykerr]

#2788 adds support for IUnknown - is that what you mean?

[mwcampbell]

Yes, thanks.