[web] generalize focusability in semantics

[yjbanov]

Introduce 2 new classes for a11y focus management:

• AccessibilityFocusManager: a generic class that attaches "focus" and "blur" event handlers, and forwards the events to the framework as SemanticsAction.didGainAccessibilityFocus and SemanticsAction.didLoseAccessibilityFocus respectively. Provides the changeFocus method for the framework to move a11y focus to the target element.
• Focusable: a role manager that provides generic focus management functionality to SemanticsObjects that don't need anything special.

Rewrites focus management using the above two classes as follows:

• All focusable nodes except text fields and incrementables get the Focusable role (all custom focus stuff in Tappable was removed and delegated to Focusable).
• Incrementable uses a custom <input> internally and so it cannot use the Focusable role. Instead, it uses AccessibilityFocusManager to manage the focus on the <input> element.

Behavioral changes:

• Fixes [Web] Setting autofocus on checkbox doesn't have an effect in accessibility mode flutter/flutter#118737, but more generally fixes all nodes that use the isFocusable and hasFocus bits.
• Tappable only partially implemented focusability (e.g. it didn't generate the respective SemanticsAction events). Now by delegating to Focusable, it will inherit all the functionality.
• Incrementable and Checkable (checkboxes, radios, switches) get focus management features for the first time.
• Elements that are not inherently focusable (text, images) can now be focused if semantics requires them to be.
• TextField is left alone for now as focus and on-screen keyboard interact with each other in non-obvious ways.