[infra] updated packaging config

This updates the .npmignore file to ignore some new files that were
recently added, and to keep the `core` directory, so that noVNC can be
used as an ES6 module from NPM.

This also updates package.json to clean when generating lib files in the
`prepare` phase (which has replaced the `prepublish` phase).

.gitignore

@@ -9,3 +9,4 @@ utils/websockify
 recordings
 *.swp
 *~
+noVNC-*.tgz