HLSL: Flesh out misc. declaration grammar: semantics/registers/annotations/precise/etc.

Details within these bear even more fleshing out, but would like to have
that driven by actual need.

Test/hlsl.float4.frag

@@ -1,5 +1,10 @@
 float4 AmbientColor = float4(1, 0.5, 0, 1);
 
+bool ff1 : SV_IsFrontFace;
+float4 ff2 : packoffset(c0.y);
+float4 ff3 : packoffset(c0.y) : register(ps_5_0, s[0]) ;
+float4 ff4 : VPOS : packoffset(c0.y) : register(ps_5_0, s[0]) <int bambam=30;> ;
+
 float4 ShaderFunction(float4 input) : COLOR0
 {
     return input * AmbientColor;