Add file watching example

It would be beneficial to add a file watching example to help show people how to keep a project up to date with the file system.

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I recommend using file watchers such as [chokidar](https://github.com/paulmillr/chokidar) alongside this.

Some methods that are helpful:

* `SourceFile#refreshFromFileSystem()`
* `SourceFile#remove()` - Removes a file from the project (not from the file system)
* `Directory#remove()` - Removes a directory from the project (not from the file system)
* `Project#addExistingSourceFileIfExists(filePath: string)` - Useful for adding a source file to the project, if it exists.

Working with directories is useful.