Use module variables in __init__.py for static values

[farmio]

These are marked Final now and don't need to be called (entrypoint_js instead of entrypoint_js()) which looks cleaner and is less error prone.

Also added webcomponent_name as this should live in the panel package instead of the integration.

closes #77

example use:

import knx_frontend as knx_panel

    if DOMAIN not in hass.data.get("frontend_panels", {}):
        hass.http.register_static_path(
            URL_BASE,
            path=knx_panel.locate_dir(),
            cache_headers=knx_panel.is_prod_build,
        )
        await panel_custom.async_register_panel(
            hass=hass,
            frontend_url_path=DOMAIN,
            webcomponent_name=knx_panel.webcomponent_name,
            sidebar_title=DOMAIN.upper(),
            sidebar_icon="mdi:bus-electric",
            module_url=f"{URL_BASE}/{knx_panel.entrypoint_js}",
            embed_iframe=True,
            require_admin=True,
        )