Issue with core-js package during stackblitz example builds

[daniloff200]

Bug description:

Right now, if you open any stackblitz demo from the doc page, by pressing at the stackblitz icon near the demo section anchor, you will see an issue, related to core-js, which was not installed

Plunker/StackBlitz that reproduces the issue:
Basically any stackblitz example, like here : https://valor-software.com/ngx-bootstrap/#/alerts#basic

Versions of ngx-bootstrap, Angular, and Bootstrap:

ngx-bootstrap: 6.1.0 and earlier versions

Angular: 9 and 10, maybe 8 too

Expected behavior

No issues during stackblitz example build

[daniloff200]

According to stackblitz/angular-cli-template#9, stackblitz will install 3+ version of core-js by default, and, you have to remove it and use, for example, v2.6, and the issue will be fixed. But that's not cool to do that every time :)

[daniloff200]

Thanks to @sibiraj-sr and his comment here, we have a way to fix it, by updating the core-js version to 3+, and, changing the imports stackblitz/core#930 (comment)