Spell checker

It's not normal to use spell checkers when programming, but my students are convincing me it might be a good idea!

This is a little bit tricky, because there are definitely cases where you intend to spell something differently. but I think it could work, with some combination of:
1. Break apart camel case before checking spelling.
2. Don't be picky about capitalization or punctuation (e.g., accept "dont" for don't, etc.)
3. Exempt any built-in functions that don't form words, like `fst`.
4. Provide a way to add new words to the dictionary... but for this project only! Each project would have a custom dictionary.

The issue names no files, tests, or entry points. Start by reviewing the proposed spelling rules and locating the CodeWorld editor or language-tooling entry point. Define the intended scope for token handling, exemptions, and project-specific dictionary words before implementation; done should include a usable spell-checking feature with coverage for those cases.

Written by the indexing model from the issue text.