feat: add suppressInternalGestures flag to disable built-in input

[F1orian]

What

Adds an explicit bool suppressInternalGestures (default false) to CanvasKit. When true, the widget builds no internal pan/pinch GestureDetector and ignores pointer-signal (mouse-wheel) zoom — regardless of interactionMode or gestureOverlayBuilder.

Why

Today the only way to fully suppress canvas_kit's own input handling is to be in programmatic mode and pass a non-null gestureOverlayBuilder. But apps that own all input from their own gesture layer — stacked as a sibling above CanvasKit so it can hit-test in world space and win pointers over the items — don't actually want canvas_kit's gestureOverlayBuilder, because that builder's result is composited below the item children (it can't host a layer that must hit-test above tiles).

So those apps currently pass an empty gestureOverlayBuilder: (_, _) => SizedBox.shrink() purely to flip the internal switch. That's a non-obvious idiom — a builder's mere presence silently changes gesture behavior, with no compile-time or runtime signal of intent.

This flag makes "the embedding app owns all input" explicit and self-documenting, and decouples it from "I'm supplying an overlay."

Behavior / compatibility

• Default false → no change for existing callers.
• The previous implicit suppression (programmatic + non-null gestureOverlayBuilder) still applies, so existing code keeps working unchanged.
• Both the internal GestureDetector and the onPointerSignal wheel-zoom are gated on the same computed condition.

Tests

Adds the package's first test/ directory (suppress_internal_gestures_test.dart):

• control: a default CanvasKit pans on drag (proves the harness drives the gesture, so the assertion below isn't vacuous);
• suppressed: with suppressInternalGestures: true, the library does not move the camera on drag.

Notes

The diff is intentionally minimal and additive: one field, one constructor parameter, and two one-line gate changes in build(). Happy to adjust the name (suppressInternalGestures vs. e.g. handleInput: false) to match your preference.

[vento007]

noticed.. will not get time before the weekend to check your PRs, thx :)