Move babel-polyfill to be the first thing in Webpack's entry

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• moves babel-polyfill as first thing in the Webpack's entry.

Relevant technical choices:

Ensuring the transformations made by babel-polyfill happen first, makes Internet Explorer a bit more happy. Inspired by this comment and following ones: facebook/react#8379 (comment)

Test instructions

This PR should be carefully tested on a Windows machine with IE11 and also on mac with other browsers to make sure there are no regressions.

• set up a dev environment on a Windows machine ( 😬)
• switch to this branch
• build the JS: building on Windows you will probably get the error mentioned in Yoast SEO 8.0 does not work in IE11 #10539 (comment) and you need to manually install yarn add babel-preset-stage-0
• edit a post using Internet Explorer 11
• verify the metabox renders and works correctly, both in the Classic Editor and in Gutenberg
• try the two analysis to check they work
• disable / enable features to see the metabox content changes
• etc.

Important notes:

• in the Classic Editor, with IE11 TinyMCE is just a blank area; seems unrelated and will open a separate issue
• in Gutenberg, with IE11 the metabox position is broken and covers the top part of the post content; this must be reported upstream
• with IE11, especially when the browser console is open, I've got a few browser crashes with no apparent JS errors; it's just my personal opinion but seems to me there's just too much JavaScript to process for IE

Fixes #10539

[jcomack]

CR done 👍

[jcomack]

Acceptance done 👍

@dariaknl is currently testing in IE11 to ensure everything is truly fixed. Once I get the all clear from her, I'll merge this PR.