Use new format, closer to devfile v2, to specify Che Theia plugins in addition to meta.yaml files

= Using a new format to specify editors and plug-ins Before this update, {prod-short} used the `meta.yaml` format. With this update, {prod-short} is using: * Devfile v2 format to specify editors and plug-ins * an additional `che-theia-plugin.yaml` fragment to specify plug-ins preferences This format enables to define: * dependencies between plug-ins * sidecar options * plug-in preferences