Expose NSFW to inheritors of NsfwFilesystemWatcher

[mcgordonite]

What it does

This PR adds a protected method to NsfwFileSystemWatcherServer a new service identifier NsfwOptions that allows extensions to specify options to pass to nsfw.

For example, this enables extensions to specify the debounceMS setting for file watchers.

How to test

To test that the options are sent to nsfw, you could:

1. Rebind NsfwOptions to { debouncMS: 10000 } in a backend extension.
2. Build the extension package and browser example app.
3. Start the browser example app.
4. Try to add multiple files to the workspace in quick succession. Verify that there is a visible debounce in updating the file navigator.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[paul-marechal]

I know this is a minimal modification, but would it be easier to return the complete option object from this new method? Extenders would have to return something like:

    protected getNsfwOptions(watcherId: number, basePath: string, options: WatchOptions): nsfw.Options {
        return Object.assign(super.getNsfwOptions(watcherId, basePath, options), {
            // ...
        });
    }

WDYT?

[akosyakov]

I wonder how it can be extended, we run nsfw watching in separate process without DI to avoid blocking the server process.

[mcgordonite]

@akosyakov and @marechal-p, sorry for the delay getting back to you.

@akosyakov was right, it was hard to override the file watcher server correctly. Extenders would have had to overwrite the nsfw-watcher sub-processes entry script as well.

Instead, I've updated this PR to add an NsfwOptions service identifier, so other extensions can rebind their new options:

rebind(NsfwOptions).toConstantValue({
    debounceMS: 5000
});

For full disclosure, we have forked nsfw to add some application-specific behaviour that is controlled by a new flag in nsfw.Options.

[akosyakov]

code looks clean, watching still works