feat #1229 watch task for rebuilding the bootstrap code

This commit adds a watch task (runnable via `npm run watch`) which watches
all the files in `src` folder and runs the bootstrap build whenever any of
them changes.

This is meant for developer's convenience (re-running tests in the
browser in the development cycle) until JIRA-403 is
implemented.

Since 1.6.1, running tests locally requires a bootstrap build
which was added with the migration to noder-js (commit
7743a70ed54bcdf18aba2ecb2c0600a5e1142ab7).

Close #1229.