Expose API to allow other extensions depending on this one register custom watches

[vaclavHala]

Hi, it'd be neat to allow programmatic registration of ICommands.

We have an extension which needs to do some file watching and rather than basically rewriting what you have here from scratch we'd like to do something like the following:

// code in activate() of our extension
let watcher = extensions.getExtension('filewatcher').exports; // also possibly activate the ext here
watcher.register({
  "match": "*.foo",
  "isAsync": true,
  "cmd": "echo '${file} bar'",
  "event": "onFileChange"
})

I haven't fully thought through how to show to the user what commands were registered this way. I guess there are multiple options - e.g. have some read-only view which would expose these, or dump these to some config file directly (probably not the user config).

Is this something you'd consider having in your extension? I'd be willing to contribute the patch if you agre to go through with this idea