Remove support for the class keyword

[yorickpeterse]

As part of #779 we're introducing the type keyword to replace the class keyword in 0.18.0, while still supporting both. In 0.19.0 we'll drop support for the class keyword entirely.

The following changes need to be made:

• Remove support for class in inko fmt
• Remove support for the classes field in idoc in favour of only supporting (and requiring) the types field
• Remove parser support for the class keyword
• Remove support for class in the Vim syntax file
• Remove support for class in the Tree sitter grammar
• Remove support for the class keyword in the VS Code grammar