Add Watch Mode #19

Although most of the infrastructure is complete, there lacks a way to set up a watch mode, where plugins are iteratively built when file changes are detected. For a single installable plugin, yarn workspace <plugin> run build --watch may work. However, that doesn't work for

1. external plugins where the files need to be copied to the dist folder
2. multiple plugins

An example could be

yarn watch @sourceacademy/common-tabs @sourceacademy/runner-test