feat: add shadow dom support, fixes #321

[enkeli]

When using angular-editor under a shadow dom encapsulated component, it was throwing errors as it tried to find some elements by their id's using the global document api:

This pull request tries to solve it by changing the way we access those elements using Angular's ElementRef instead of document.
It also adds a new page on the angular-editor-app project to showcase the use of angular-editor under a shadow dom encapsulated component.

Test are passing:

[codecov]

Codecov Report

Merging #479 (c2cfe4b) into master (40431f4) will increase coverage by 0.24%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
+ Coverage   20.66%   20.91%   +0.24%     
==========================================
  Files           8        8              
  Lines         479      483       +4     
  Branches      103      103              
==========================================
+ Hits           99      101       +2     
- Misses        377      379       +2     
  Partials        3        3              

Impacted Files	Coverage Δ
...editor/src/lib/angular-editor-toolbar.component.ts	18.25% <42.85%> (+0.51%)	⬆️
...angular-editor/src/lib/angular-editor.component.ts	17.44% <60.00%> (+0.38%)	⬆️

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[akib1997]

The same issue is happening to me. Does anyone have a solution?