filesystem watcher for external changes

[samuelstroschein]

Problem

getMessages() needs to react to external filesystem changes.

Proposal

1. Add a filesystem watcher to createMemoryFs
2. Inlang instance keeps tracks what file a plugin reads with getMessages
3. Inlang instance establishes a watcher for the files in (2)
4. Upon changes on any file, re-execute getMessages

Performance should be good enough/can be improved in the future.

[samuelstroschein]

Related #839

[samuelstroschein]

Implemented