Improve iOS fidelity of `barrierColor`s and edge decorations for full-screen Cupertino page transitions (#95537)

This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.

I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.

I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.

This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.

* Create state restoration templates, 

* Update existing state restoration samples to use the new templates

* Add to README, decided to require stateful widgets to include RestorationMixin

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

* Expose CupertinoModalPopupRoute for state restoration

* Add state restoration test, expose kCupertinoModalBarrierColor

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

* Remove nullOk from cupertino color resolution

* Add CupertinoDynamicColor.maybeResolveFrom

* Remove CupertinoDynamicColor.maybeResolveFrom again

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

This reverts commit da58f5e6.

* migrate some cupertino files to nullsafety

* address review comments

* Add material page, cupertino page, and transition page classes

* update

* update comments

* fix test

* addressing comments

* make page getter private

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag