Support multi line character `_` in language constructs

[serkonda7]

Also, I realize that while we are at it, we should also support multi-line comments that make use of the line continuation character (_) as well:

' Multi-line _ 
  comment

Originally posted by @DecimalTurn in #81 (comment)

Note: This should theoretically affect other grammar rules as well.

[h8nor]

#83 (comment)