fix(polyfills): move polyfills to own entry point

[filipesilva]

Polyfills found in polyfills.ts would not be available for scripts due to being loaded in the main bundle only.

This PR loads polyfills as a separate entry point, before everything else.

Extra documentation also added to polyfills.ts.

Fix #2752
Fix #3309
Fix #4140

[cebor]

Webpack also accepts modules/package names as entry option.

entry: {
  polyfills: [
    'core-js/es6',
     'core-js/es7/reflect',
     'zone.js/dist/zone'
  ],
...
}

Would not this be a cleaner solution?

[filipesilva]

@cebor it's true that webpack accepts that, but the purpose of the scripts array is to provide feature parity with script tags. Script tags don't accept package names. I suppose the feature could be enhanced but it doesn't seem too important.

[hansl]

(cf. #3999)

While you’re at refactoring the polyfills and how they’re included, could you include the long stack trace when using debug? Would fix #2233.

[filipesilva]

@hansl see #2233 (comment), adding the long stack trace is not the solution to that issue.

[filipesilva]

Also need to add a lot of comments to polyfills to show opt-in ones for each browser.

[hansl]

LGTM, Thanks!

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