[HLSL] Add support for lexing floating-point literals in `RootSignatureLexer`

[inbelic]

This issue tracks adding support for adding literal floating point support into LexNumber of RootSignatureLexer.

There is some care need to be taken here for checking the supported floating-point formats match those from DXC.

Dependent on:

AC:

• Update LexNumber to allow support for floating-point characters
• Add unit-testing for lexed floats and any newly defined diagnostics