Support on-screen keyboard

[fujidaiti]

This PR includes changes and new features to address the issue of the keyboard overlap on the sheet. Once the PR is merged, the framework will automatically offset the sheet by the keyboard height to prevent the content from being overlapped by the keyboard.

Changes made

Breaking changes

• Change the type of SheetMetrics.viewportDimensions from Size to ViewportDimensions, which holds the MediaQueryData.viewInsets of the window (typically the keyboard height) in addition to the viewport size.

Internal changes

• Change SheetViewport to gradually offset the sheet by the keyboard height.
• Split _ContentScrollDrivenSheetActivity into the 3 parts according to the lifecycle; _ContentIdleScrollSheetActivity, _ContentUserScrollSheetActivity and _ContentBallisticScrollSheetActivity.
• Change some sheet activities such as UserDragSheetActivity and _ContentUserScrollSheetActivity to keep the visual position of the sheet unchanged while a keyboard dismissing/appearing animation is running.

New features

• Add resizeToAvoidBottomInset property to SheetContentScaffold.
• Fixes Add a way to dismiss the on-screen keyboard when the sheet is dragged #8.

Documentation

• Fixes Add a showcase that uses TextFields in a sheet #2.
• Add a tutorial for SheetKeyboardDismissBehavior.
• Add the description of SheetKeyboardDismissBehavior to the README.