Align Node server validation with Java server (async live validation)

[martin-fleck-at]

What it does

See also eclipse-glsp/glsp-server#247
Fixes eclipse-glsp/glsp#1408

How to test

• Add breakpoint during validation and see that the diagram still shows up with the 'Validating model...' message being present.

Follow-ups

Changelog

• This PR should be mentioned in the changelog
• This PR introduces a breaking change (if yes, provide more details below for the changelog and the migration guide)

[tortmayr]

In general the changes look good to me.
However, I'm not the biggest fan of pulling in additional libraries just to use one function.
Especially if its something like debounce which we can easily implement ourself.
I would be in favor of replacing the loadash import with a custom debounce function.

[martin-fleck-at]

@tortmayr I agree that pulling in libraries for a single function is not always the best option. However, we already use lodash in the client and in the theia-integration. Do you really think the impact is that large? I'm always a bit worried to re-implement such a common pattern. Note that we can already see the lodash dependency in the yarn.lock because we have concurrently in our dev (!) dependencies.