Add go_router_builder

[kevmoo]

Fixes https://github.com/csells/go_router/issues/66

Adds two new classes to pkg:go_router

[kevmoo]

• [x] fix docs related to optional params
• [x] Add error about using nullable required args - must not be nullable!
• [x] query param names (keys) as kebab-case
• [x] add back pageBuilder – need to expose random things
• [x] expando idea – implemented.
• [x] Move example under go_router_builder
• [x] support for
  • [x] num
  • [x] int
  • [x] bool
  • [x] DateTime
  • [x] Duration - punting on this. Requires double decoding/encoding. Folks should just use int
  • [x] BigInt
  • [x] double
  • [x] Uri
  • [x] Enum (kebab-case)
  • [x] String
• [x] top-level route thing (ON HOLD! I have to re-architect things to make this work!)

[chunhtai]

Since this package has been moved to https://github.com/flutter/packages/tree/main/packages/go_router, can you create a pull request against the flutter/packages instead?