SharpLab fixes: Wave 3

[ashmind]

Unfortunately these are quite hard to disentangle into separate branches, so I had to create it as a single PR.
On the positive side, this time it has tests.

Summary of changes:

1. Resolved issue with error location being outside of the bounds of the document in certain cases (when near end of file)
2. Corrected top-level duplicate methods to be reported as an error instead of an (unintended) exception
3. Corrected duplicate fields to be reported as an error instead of an (unintended) exception
4. Added a way to override direct filesystem access when referencing a manifest resource (potential security concern for Web Applications)
5. Some refactoring to allow 4, e.g. added a DriverSettings class