Champion "allow digit separator after 0b or 0x" (C# 7.2, VB 15.5)

[gafter]

• Proposal added https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/leading-separator.md
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

(from dotnet/roslyn#12680)

See PR dotnet/roslyn#20449 (implemented in 15.5)

[gafter]

Thanks to @alrz for implementing this feature!

[gafter]

I added

https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/leading-separator.md

which will serve as a spec.