Add manifest.json, versions.json of Obsidian plugin to project root

- Obsidian provides limited support for plugins in larger repositories.
  Currently, it does not have a way to specify the directory of a plugin
  So it expects the plugins `manifest.json' and `versions.json' to be at
  project root

- While this unnecessarily litters the codebase. It is the (current)
  required tradeoff for keeping the core plugins in a mono repo

versions.json

@@ -0,0 +1,3 @@
+{
+    "0.2.1": "0.15.0"
+}