[BUG] Underscores as digit separators are not supported in the grammar

[mmanela]

Describe the bug

Hack documentation says you can use underscores as digit separators but the grammar errors if you use them.

Requirements

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

To Reproduce

Use an integer with underscore digit separators

 $a = 123_456;

Which currently parses as the following (notice the ERROR)

(expression_statement
        (binary_expression
          left: (variable)
          (ERROR
            (integer))
          right: (qualified_identifier
            (identifier))))

Expected behavior

I expect usage of the underscore digit separator to parse properly.

[Saichandra2520]

I'm currently working on this issue and will submit a pull request soon. #72