Simple static location sharing (#7135)

Adds maplibre as a dependency, and behind a labs flag, lets users send and receive [MSC3488](https://github.com/matrix-org/matrix-doc/blob/matthew/location/proposals/3488-location.md) style location shares - with backwards compatibility with old school `m.location` `msgtype` location shares too.

For this to work, you have to define a valid maptile server and API in your config.json's `map_style_url`.

res/css/views/rooms/_MessageComposer.scss

@@ -256,6 +256,10 @@ limitations under the License.
     mask-image: url('$(res)/img/element-icons/room/composer/emoji.svg');
 }
 
+.mx_MessageComposer_location::before {
+    mask-image: url('$(res)/img/element-icons/room/composer/location.svg');
+}
+
 .mx_MessageComposer_stickers::before {
     mask-image: url('$(res)/img/element-icons/room/composer/sticker.svg');
 }