L.TileLayer.WMS support

[andrewgryan]

Add support for <l-tile-layer-wms> Custom Element.

• Use light DOM in accordance with existing Custom Elements.
• Support non-WMS query parameters by leveraging the underlying L.TileLayer.WMS options object
• Add unit tests to cover lifecycle events
• Add API docs to instruct users to use attributes correctly

Useful links

• Leaflet TileLayer.WMS API docs
• src/index.js to register new element
• src/l-tile-layer.test.js inspiration for tests
• src/l-tile-layer.js inspiration for new custom element

[andrewgryan]

Potential design to support non-standard WMS parameters.
Use JSON in a dedicated options attribute.

<l-tile-layer-wms
    base-url="/wms/path"
    layers="wms_layer_name"
    styles="wms_style"
    options='{"key": "value"}'
></l-tile-layer-wms>

Internal to the component, the options and standard attributes can be parsed and combined.

// connectedCallback
// Error handling omitted

const layers = this.getAttribute("layers")
const styles = this.getAttribute("styles")
const options = JSON.parse(
    this.getAttribute("options")
)

this.layer = TileLayer.WMS(baseUrl,
    { layers, styles, ...options}
)