Make plugin work for legacy users as it did before (#6)

If we want to release this plugin to the same plugin repo that the old repo was in, we should make sure users who upgrade the plugin but don't update warp don't just loose the functionality. To support this, this PR adds back the legacy plugin's hooks, and we use those hooks when we can see that the user is in warp but their warp version has not declared a version of notifications that it supports (i.e. they haven't updated). This ensures complete backwards compatibility.

demo: https://www.loom.com/share/a2f0dd9f94aa4b85881215444a6106d2