Should we Further Relax Underscores in Identifiers?

[InsertCreativityHere]

As of #1631, we now allow users to use underscores in their identifiers without needing a special metadata.
But I think we could go further. The parser still disallows:

• trailing underscores: cool_
• leading underscores: _cool
• multiple underscores in a row: cool__stuff

I could see a case for keeping the restriction on leading underscores, since that has a special meaning in some languages.
But restricting trailing and double underscores just seems unnecessary to me. I agree they're weird, but why should the parser care? Removing these restrictions is backwards compatible, and would let us remove and simply more logic in the scanner.

[pepone]

I think double underscores are reserved in C++ https://en.cppreference.com/w/cpp/language/identifiers

[bernardnormier]

We should keep these remaining restrictions.