Add onLoaded function to MMGIS API

[ac-61]

This adds an onLoaded function to the MMGIS API, which calls the input function once MMGIS has finished loading.

An example of how to call the onLoaded function:

window.mmgisAPI.onLoaded(() => { 
    // Add listener with MMGIS API now that MMGIS has finished loading
    function listener() {
        const featuresContained = window.mmgisAPI.featuresContained()
        console.log('featuresContained', featuresContained)
        const activeFeature = window.mmgisAPI.getActiveFeature()
        console.log('activeFeature', activeFeature)
    }
    window.mmgisAPI.addEventListener('onPan', listener)
})

[tariqksoliman]

Looks good now!