[HLSL] Implement Root Signature Parsing of Root Constants

[inbelic]

This issues handles defining RootConstants in-memory representation and the extra keyword/enum tokens required for lexing. Then we implement a ParseConstants method to construct the struct using previously defined parse methods.

Dependent on #126569

AC:

• Define the required keyword/enums for the Lexer
• Define the parser method
• Define the in-memory representation of RootConstants
• Add additional unit testing

[inbelic]

Small NFC change to do: #136747 (comment)