Switch lerna to independent versioning

Stops forced major bumps of unchanged packages when only one package
ships a breaking change (e.g. the recent html-to-structured-text v6,
which dragged 8 sibling packages to 6.0.0 for nothing).

From the next `lerna publish` onward each package versions on its own
and tags become `<package>@x.y.z`.

Author: stefanoverna

lerna.json

@@ -1,4 +1,4 @@
 {
   "packages": ["packages/*"],
-  "version": "6.0.0"
+  "version": "independent"
 }