Add support for declarative playback using `end` field

There are certain cases where it would be nice to just set the `end` value for a set of tweens, and have the tweens run when that end value changes.

For example:
```
GridView(
children: imageWidgets.map((i){
bool selected = imageWidgets.indexOf(i) == selectedIndex;
return i.animate().scale(
duration: 200.ms,
curve: Curves.easeOut,
begin: 1,
end: selected? 1.2 : 1);
})
```
We could then easily create this effect where the selected image scales in, while the previously selected image scales out. The others do nothing.

https://user-images.githubusercontent.com/736973/188086611-fff77915-6be6-4236-9b8e-54700c8533a3.mp4

Currently the easiest way to do this is still with the built in `TweenAnimationBuilder`, which is a fine API, but it cant hook into any of `Animate`s effects:
```
imageWidgets.map((i){
bool selected = imageWidgets.indexOf(i) == selectedIndex;

return TweenAnimationBuilder(
tween: Tween(begin: 1, end: selected ? 1.15 : 1),
duration: 200.ms,
curve: Curves.easeOut,
builder: (_, value, child) => Transform.scale(scale: value, child: img),
)
}
```
If you matched the declarative behavior of TAB, then the old `end` would become the new `begin` if `end` ever changes, which is quite nice for continuous effects (go from A > B > A). Changes to `begin` are ignored until the widget state is reloaded.

Start by comparing the requested declarative behavior with Flutter's TweenAnimationBuilder and the existing Animate effects. Verify how an end-value change should transition from the old end to the new end, including repeated changes such as A > B > A, while ignoring begin changes until widget state reloads. Done means end-driven playback works with Animate effects and matches the provided scaling example.

Written by the indexing model from the issue text.