feat(nebular): Added nebular support

[vijhhh2]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature.

What is the current behavior? (You can also link to an open issue here)
N/A

What is the new behavior (if this is a feature change)?
This PR aims to implement support for the Nebular UI for Angular.

Please check if the PR fulfills these requirements

• The commit messages follow our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• A unit test has been written for this change.
• Running npm run build produced a successful build. (Unit testing can be done by running npm test;)
• My code has been linted. (npm run lint to do this. npm run build will fail if there are files not linted.)

Please provide a screenshot of this feature before and after your code changes, if applicable.

Other information:

[netlify]

✅ Deploy Preview for formly-dev ready!

Name	Link
🔨 Latest commit	b48f11f
🔍 Latest deploy log	https://app.netlify.com/sites/formly-dev/deploys/62b95f992437a80008d4e6f4
😎 Deploy Preview	https://deploy-preview-3360--formly-dev.netlify.app/
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[MaxKless]

Super cool that you built an entire new theme for formly!
I'm wondering, thinking about the long-term health and maintainability of formly, is it smart to add more stuff to the main formly repo?
I know that @aitboudad has already said he'd do it but I feel like he's very busy these days and adding more code might be a burden that distracts from core formly maintenance and development.

Especially since there is already a great package that can simply be plugged into any project.

[vijhhh2]

I agree with @MaxKless on this

[MaxKless]

So, should we close this, @aitboudad @vijhhh2 ?

[aitboudad]

I tend to agree with some points here, I will think about it after finishing with V6 release 🙏!