Optionally watch preloaded code

If I preload a file, it’d be nice if the file could be re-evaluated every time it changes. This is similar to auto-reloading modules, but slightly different. It should be easy to implement and would be tremendously useful for combining a text editor with nesh as a REPL.

No file or test is named. Start by tracing how nesh preloads files and compare that path with its auto-reloading module behavior. Done means an optionally watched preloaded file is re-evaluated after each change for the text-editor/REPL workflow, while ordinary preloading remains available.

Written by the indexing model from the issue text.