Add CustomMessage component

<template>

  <l-map ref="map" :zoom="2" :center="[47.41322, -1.219482]">

    <l-tile-layer

      url="https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png"

      layer-type="base"

      name="OpenStreetMap"

    ></l-tile-layer>

    <l-control class="leaflet-control leaflet-demo-control"

      >Hello, Map!</l-control

    >

  </l-map>

</template>

<script>

import { LMap, LTileLayer, LControl } from "./../../components";

export default {

  components: {

    LMap,

    LTileLayer,

    LControl,

  },

};

</script>

<style>

.leaflet-demo-control {

  background: white;

  border: 1px solid steelblue;

  border-radius: 0.6em;

  padding: 0.5em;

}

</style>