Add support for fill, weight, grade, and optical size to `Icon` (#106896)

* wip

* update documentation

* x

* remove trailing spaces

* x

* remove useless CupertinoIconThemeData copyWith override

* add tests

* remove trailing spaces

* fix isConcrete

* x

* x

* x

* remove trailing spaces

* tweak docs

* mention that font filenames often indicate the supported axes

* add back cupertino IconThemeData copyWith

* update copyWith