Apply best practice to spelling rules

[aireilly]

We should apply @jdkato's advice for improving our spelling rules.

Filters should be used for ignoring large classes of words that you expect to be false positives. See the built-in ones for examples.

Case-insensitive exceptions should be in a dictionary.

Case-sensitive exceptions should be in a vocabulary, providing the added benefit of ensuring they're ignored during spell check but always capitalized correctly.

Context: #900