fix: add findActiveAnchor fallback for bound flicking

[malangfox]

Issue

#850

https://codepen.io/malangfox/pen/WNmPemg
Pressing the moveTo button, opening the console, and dragging the flicking should reproduce it.

Details

Position is not reachable error occurs when using the moveTo method for the first panel with bound: true.

In the above demo, when 4 Panels are placed, the total number of Anchors is 2.

This is because the Panel labeled 1 and the Panel labeled 2 share the same Anchor point.
Similarly, the panel with 3 and the panel with 4 share the same anchor point.

This is because there is no way to send Panels 1 / 4 to the center with the bound option enabled, which removes the left and right empty spaces.

This means that in this situation, only the Anchor points of Panel 2 and Panel 3 exist.

If we use the moveTo method with Panel 1, the Anchor point we stop at is the Anchor of Panel 2, but the index of the active Panel in the flicking will be the first Panel.

In this situation, moveToPosition, which calculates the arrival point at the end of a drag with moveType: "snap", has the following logic.

const activeAnchor = camera.findActiveAnchor();
const anchorAtCamera = camera.findNearestAnchor(camera.position);
if (!activeAnchor || !anchorAtCamera) {
  return Promise.reject(new FlickingError(...));
}

and findActiveAnchor behaves as follows.

public findActiveAnchor(): AnchorPoint | null {
  const flicking = getFlickingAttached(this._flicking);
  const activeIndex = flicking.control.activeIndex;

  return find(this._anchors, anchor => anchor.panel.index === activeIndex);
}

The activeIndex is Panel 1, but none of the anchors corresponds to Panel 1, so it returns null.

Since activeAnchor is null, an error message is raised via Promise.reject.

I handled the exception of the findActiveAnchor by returning the closest Anchor to the active panel's position as a fallback if no corresponding Anchor for the active panel exists.

[coveralls]

coverage: 82.719% (+0.08%) from 82.638%
when pulling e58557a on malangfox:fix/snapcontrol-findActiveAnchor
into 3d52804 on naver:master.

[daybrush]

LGTM