Refactor allowed syntax checks for attributes

[Scott-Guest]

Fixes #3982

Previously, each attribute specified an allowed set of Class[_]s where that attribute may occur. However, both a cell and a non-cell are just Productions, so no Class[_] restriction is sufficient to distinguish cells.

This PR then refactors to instead have an allowed set of Att.Syntax types, each with a isA(term: AnyRef): Boolean method to check membership.

In process, we also:

• Restrict label(_) to rules, claims, contexts, and context aliases rather than allowing it on any sentence
• Restrict group(_) to productions
• Restrict cell, cellName, exit, stream, multiplicity, and type to cells

[Scott-Guest]

Parking this until we can move CheckAtt to immediately after parsing