Syntax serialization simplification

[keyboardDrummer]

What was changed?

• Process a few comments from https://github.com/dafny-lang/dafny/pull/6263f
• Generate the schema and deserialization code, which produces a few changes because of existing changes in master (related to Function and Predicate), and a few changes coming from the above changes, related to the "traits" parameter.
• Simplify serialization related generator code by no longer looking at fields and properties.

How has this been tested?

• Changes covered by existing tests

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[alex-chew]

The changes look good to me, although some CI failures suggest that nulls are still sneaking into TopLevelDeclWithMembers.Traits (e.g. https://github.com/dafny-lang/dafny/actions/runs/15606784506/job/43958075689?pr=6270#step:10:15)