Heredoc identifier can accept any starting identifier value

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Typically, all identifiers must start with an ASCII letter or underscore and proceeding alphanumeric character, however, numeric and underscore-only identifiers are accepted for heredocs:

p <<-123
  4567
      654
  123
# => "4567\n    654"

p <<-_
  foo
  bar
  _
# => "foo\nbar"