Give better error message on consecutive numeric separators

[DanielRosenwasser]

From #20324 (comment), and contingent on #20324 first getting merged.

let x = 123__456;

Note the double underscores.

Actual: Numeric separators are not allowed here.

• A numeric literal cannot contain multiple consecutive separators.
• Multiple consecutive numeric separators are not permitted.

[harunurhan]

I'll take this one as my first, if nobody else is working on it.