Add constant support for windows-rdl

[kennykerr]

Building on #3861, this update adds parsing support for Win32-style constants. Here's an example of a .rdl definition:

mod Sample {
    const D3D12_MAJOR_VERSION: u32 = 12;
    const D3D12_MINOR_VERSION: u32 = 0;
}

Much like #3873, ECMA-335 doesn't support what it calls "fields" without an associated type. Win32-style constants are thus also added to the per-namespace "Apis" type:

.class public auto ansi sealed Sample.Apis
	extends [mscorlib]System.Object
{
	.field public static literal uint32 D3D12_MAJOR_VERSION = uint32(12)
	.field public static literal uint32 D3D12_MINOR_VERSION = uint32(0)

}

So far it only handles integer constants. Support for string and struct literal constants coming soon.