A heredoc with interpolation containing [ ] in the expression can cause syntax error

Heredoc expressions with interpolation using an access expression like `$facts['somefact']` could fail with a syntax error depending on the relative location in the source file of the heredoc and surrounding whitespace (anywhere) in the source file. This was caused by a faulty calculation of location of interpolated expressions which also showed up as the wrong line/position in error messages related to the interpolated expressions. The handling of the position of so called "sublocated expressions" is now rewritten and the problem with seemingly random "syntax error" from the use of `[]` expression in interpolation is gone. Advanced: Anyone having written tools that walk the AST tree should note that the `SublocatedExpression` will no longer be generated by the parser as all nodes under that node are now immediately updated with their correct offset and length. Although the `SublocatedExpression` class itself will not be removed until Puppet 7.0.0, it will no longer appear in AST produced by the parser.