[a11y-base] Move long touch gesture from context-menu to the base package

[web-padawan]

Describe your motivation

Because of historical reasons, vaadin-context-menu registers a Polymer gesture handler to deal with iOS:

web-components/packages/context-menu/src/vaadin-contextmenu-event.js

Lines 60 to 73 in bc227bd

	this._timerId = setTimeout(() => {
	const ct = e.changedTouches[0];
	if (!e.shiftKey) {
	if (isIOS) {
	this._fired = true;
	this.fire(t, ct.clientX, ct.clientY);
	}
	// Needed to prevent any 'tap' gesture events from firing
	// which could potentially cancel/close the overlay.
	prevent('tap');
	}
	}, 500); // Default setting for Android and iOS.
	},

We might want to re-use this logic with other components e.g. vaadin-tooltip on mobile or something else.

Describe the solution you'd like

1. Move vaadin-contextmenu-event.js file to @vaadin/a11y-base package,
2. Try to improve test coverage, even though we don't test on iOS Simulator,
3. Check out React Aria's useLongPress hook implementation for more ideas.