Handle remaining interpolated-string forms adjacent to '=' (#16696)

[edgarfgp]

Follow-up to #19820, which fixed = adjacent to a $"..." interpolated string. That fix also covered =$"""...""" for free, but the verbatim and multi-dollar forms still lexed =$@, =@$, =$$ as the invalid operator =$.

This extends the same rule to those forms.

Before

letn=42leta=$@"{n}"// error FS0035: '$' is not permitted ... in operator namesletb=@$"{n}"// error FS0035letc=$$"""{{n}}"""// error FS0035

After

letn=42leta=$@"{n}"// "42"letb=@$"{n}"// "42"letc=$$"""{{n}}"""// "42" (multi-dollar strings use double braces for holes)

The lexer matches = plus the interpolated-string opener, consumes only the =, and rewinds so the regular interpolated-string lexer handles the rest. Operators containing $ are still rejected with FS0035, unchanged.

[github-actions]

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

[github-actions]

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: modifies src/Compiler/lex.fsl lexer rules

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