Added error on line braek in string

[GreasySlug]

Related: #167.

It turns out that the current string also works as a multi-line string.

However, this is invalid and has been fixed.

Existing tokenize tests have passed successfully.

Changes proposed in this PR:

• "match" statement is used instead of "if" statement.
• Now returns an error if '\n' is input
• Added error and hint text
• Added test to check for parse error

@mtshiba

[mtshiba]

Thanks!