Error on trailing backslash

[blakeembrey]

Closes #433. Most straightforward approach that takes minimal bytes.

An alternative might be to re-introduce the consume function that throws an error itself, and tryConsume token by token, but I removed that a while back to reduce bytes and function call overhead. The main difference would come down to the error message, which using a tryConsume style approach would hit the universal error message and print something like Unexpected end at index 9, expected char from:

Unexpected ${token.type} at index ${token.index}, expected ${endType}

I don't think adding the overhead back is worth it in this case, it's simpler to treat it as a one off error to handle.

[blakeembrey]

Within quotes are fine to increment the undefined because it's already handling the Unterminated quote error. If we wanted to be extra safe we could add the same error there, but I think it's simpler and clearer to keep the current error message.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.74%. Comparing base (7f05876) to head (b4baac3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files           1        1           
  Lines         389      392    +3     
  Branches      148      150    +2     
=======================================
+ Hits          388      391    +3     
  Misses          1        1           

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