[HLSL] Implement the Root Signature parser

[inbelic]

This issue has been changed to represent the umbrella task of implementing the Root Signature parser.

It consists of define a Lexer that will convert a root signature string into their Token representation and a Parser that will convert from the Token representation to their in-memory representation as a RootElement

It should be self-contained from the parsing of HLSLRootSignatureAttr.

Note that the parsing of the root signature will take place in SemaHLSL so we have the capability to do semantic analysis when parsing, but we will defer much of this work to issue #119019.

This issue is resolved when all sub-issues are completed.