Implement fromJson() constructors for LSP types

Also add a canParse() method that can check whether some provided JSON can be decoded into that type (used for detecting with type from a union to decode as).
Support equality checks on unions and a valueEquals() helper
Remove brittle tests that do exact comparisons on generated code
Fix deserialisation of lists to be cast and toList()'d
Add a more complete JSON test that includes lists, enums

Change-Id: Id56fdad9b1454e540e55907e0ff2608263a87c40
Reviewed-on: https://dart-review.googlesource.com/c/80580
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Danny Tuppeny <dantup@google.com>

Author: DanTup