Avoid bundling ol imports into the final build

[menghany]

No description provided.

[jonataswalker]

This is great but the iife bundle have some problems.

You'll see if you run examples/contextmenu.html. Making some tests it looks like that this change can fix it:

rollupOptions: {
    external: [/^ol.*/],
    output: {
        globals: {
            'ol/MapBrowserEvent': 'ol.MapBrowserEvent',
            'ol/control/Control': 'ol.control.Control',
        },
        assetFileNames: () => 'ol-contextmenu.css',
    },
},