Add watch handling for non-symlinked dynamic extensions

[blink1073]

Problem

Extensions that are installed with labextension develop without a symlink currently do not support watch mode.

Proposed Solution

• When we call labextension watch, check for a symlink to the watched directory. If none exists, check for a labextensions directory. If none exists, bail. If there is a labextensions directory, set up file system watchers to copy changes to that location from the src directory of the extension in addition to running the local watch command.