Consecutive underscores in number literals and number parsing methods

[straight-shoota]

Both integer literals and number parsing methods like String#to_i (with underscore: true) allow and ignore underscores in the number:

1_1                            # => 11
"1_1".to_i(underscore: true)   # => 11

The semantics are very similar but there is a difference: Literals allow multiple consecutive underscores, which is an error in String#to_i:

1__1                          # => 11
"1__1".to_i(underscore: true) # Error: Invalid Int32: 1__1

Considering the similarity of both approaches, it should be expected that both underscore semantics are identical and don't differ by such a minor detail.
(It would also help for using parsing methods in the compiler implementation.)

Originally brought up in #11196 (comment) ff.

[HertzDevil]

1__1 is a syntax error in both Ruby and Python, and in C++ where the separator is ' instead, 1''1 is also an error there.

[oprypin]

I think consecutive underscores should be disallowed.