Support continuation mark and catch invalid escape sequence

[H-DNA]

Summary

• Support continuation mark
• Catch invalid escape sequence
• Support escaping triple quotes

Issue

(issue link here)

Lasting Changes (Technical)

• Lexer: escape sequence handling
• A new compiler error for invalid escape sequence.

Checklist

Please check directly on the box once each of these are done

• [x] Documentation (if necessary)
• [X] Tests (integration test/unit test)
• [ ] Integration Tests Passed
• [ ] Code Review