"Atom" methods should include overload or metadata to indicate that an Atom is an option

Consider the [`GetPropW'](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getpropw) method, which allegedly takes a string pointer as a second parameter but documentation allows for an atom instead. From the doc:

> The atom, a 16-bit value, must be placed in the low-order word of the lpString parameter; the high-order word must be zero.

For the C# projection, this is *very* unnatural to pass an arbitrary 32-bit value (expanded from 16-bits) in as if it were a pointer. Such methods merit a friendly overload that accepts a `ushort` (to match the type returned from `GlobalAddAtomW`).

We will need an attribute on the atom-accepting pointer parameter in the metadata in order to know to generate this.