Add fuzzing tests to check for crashes

[eisenwave]

There have been a number of instances where I've encountered bugs not caught by any of the test files or unit tests, but that could have been caught by unit testing. #52 is a prime example.

In another case, it 5E resulted in an infinite loop in the HTML parser.

To start with, we should have two kinds of fuzzing tests:

1. For every ASCII character and code point within certain ranges, test if a single-character file could be highlighted with no crash.
2. For some fuzz test file, test if highlighting with no crash is possible for every incompete file up to some length.