Get rid of string enums, use union types instead

Our string enums, like those used for *EventType, are code that is redundant and only used for type checking. Instead, we could use union types that ensure types just as well, without adding payload to the build.

The easiest way to get documented event types in JSDoc while making TypeScript turn them into union types is a @typedef in JSDoc with a keyof operator in TypeScript:

/**
 * @typedef {Object} MapBrowserEventTypeKeys
 * @property {string} singleclick A true single click with no dragging and no double click. Note that this
 * event is delayed by 250 ms to ensure that it is not a double click.
 * @property {string} click A click with no dragging. A double click will fire two of this.
 * @property {string} dblclick A true double click, with no dragging.
 * @property {string} pointerdrag Triggered when a pointer is dragged.
 * @property {string} pointermove Triggered when a pointer is moved. Note that on touch devices this is
 * triggered when the map is panned, so is not the same as mousemove.
 */

/**
 * @typedef {keyof MapBrowserEventTypeKeys} MapBrowserEventType
 */

/** @type {MapBrowserEventType} */
const a = 'pointerdrag';

See also https://blog.bam.tech/developer-news/should-you-use-enums-or-union-types-in-typescript.

[simonseyock]

I am wondering if the *EventType.js files could be integrated into the *Event.js files at that go? I’d like to look into this issue after I finished https://github.com/openlayers/openlayers/pull/12430.

[simonseyock]

I like the idea of not using keyof and @fires. It will probably cause less issues with typing generation. Removing the jsdoc plugins is great as well, it makes it easier to find where the actual documentation texts are. Also it reduces the dependency on jsdoc, which is not developed much anymore.