feat(datepicker): add ivy support

[Domainv]

Steps to reproduce:
ng new hello-ivy --enable-ivy
npm i
npm i ngx-bootstrap@next
npm run build

Closes #5257

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• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated tests.
• added/updated API documentation.
• added/updated demos.

[codecov]

Codecov Report

Merging #5268 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #5268   +/-   ##
============================================
  Coverage        51.04%   51.04%           
============================================
  Files                3        3           
  Lines               96       96           
  Branches            17       17           
============================================
  Hits                49       49           
  Misses              37       37           
  Partials            10       10

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[codecov]

Codecov Report

Merging #5268 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #5268   +/-   ##
============================================
  Coverage        51.04%   51.04%           
============================================
  Files                3        3           
  Lines               96       96           
  Branches            17       17           
============================================
  Hits                49       49           
  Misses              37       37           
  Partials            10       10

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 669d7e1...2d8dfef. Read the comment docs.

[ludmilanesvitiy]

import { BsDatepickerModule } from 'ngx-bootstrap/datepicker'; - works well
but with import { BsDatepickerModule } from 'ngx-bootstrap'; - error thrown:
ERROR in Symbol BsDatepickerContainerComponent declared in /home/vs/Projects/testNGXIvy/node_modules/ngx-bootstrap/datepicker/themes/bs/bs-datepicker-container.component.d.ts is not exported from ngx-bootstrap (import into /home/vs/Projects/testNGXIvy/src/app/app.component.ts)
Looks like this is important issue, because a lot of users using just import from base module

[ludmilanesvitiy]

Tested with "angularCompilerOptions": { "enableIvy": false, and true, tested imports from ngx-bootstrap and ngx-bootstrap/module, smoke tested demo site - looks good.

[Splaktar]

Did you open (or find) an issue against ngcc with Ivy to support the spread operator?

[Domainv]

It already fixed, I checked it on angular 8.1.2
angular/angular#29835