Add Unity specific words to spell checker dictionary

[citizenmatt]

Various words from the Unity API are not recognised by the spell checker, and cause warnings. E.g. "gizmo" and "rigidbody".

These should be added as custom words to the spell checker. This can be done in two ways:

• In the backed via IDictionarySource. This will be available in both ReSharper and Rider, and might be possible to only apply when we know it's a Unity project.
• In the frontend via BundledDictionaryProvider. This will only be available in Rider, but will appear in the Spell Checker options UI, and can be disabled. The dictionary is a plain text file of words, loaded from a file which is loaded as a resource.

The list of words should probably come from reflecting over the Unity API - get all classes, methods and properties and split the names based on camel case, and then review.