Fix polyfill dependency issues on WP 4.9

[pierlon]

Summary

Fixes #6100

The wp-i18n script now depends on wp-hooks (see WordPress/gutenberg#27966), so that dependency is now polyfilled. Also, this PR ensures dependencies for the wp-api-fetch script are being registered, which they previously were not.

This PR also enqueues the polyfills before loading any assets for the amp-validation-counts handle.

Demo

20210426201040.mp4

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[codecov]

Codecov Report

Merging #6119 (e35a71b) into develop (f7ad0a8) will decrease coverage by 0.00%.
The diff coverage is 25.00%.

@@              Coverage Diff              @@
##             develop    #6119      +/-   ##
=============================================
- Coverage      75.03%   75.02%   -0.01%     
  Complexity      5829     5829              
=============================================
  Files            232      232              
  Lines          17618    17619       +1     
=============================================
  Hits           13219    13219              
- Misses          4399     4400       +1     

Flag	Coverage Δ	Complexity Δ
javascript	79.84% <ø> (ø)	0.00 <ø> (ø)
php	74.81% <25.00%> (-0.01%)	5829.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
src/Admin/ValidationCounts.php	33.33% <0.00%> (-1.67%)	4.00 <0.00> (ø)
src/Admin/Polyfills.php	23.72% <33.33%> (ø)	12.00 <0.00> (ø)

[github-actions]

Plugin builds for e35a71b are ready 🛎️!

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[delawski]

@pierlon Thank you for fixing this bug. Everything is working well for me both on 4.9.8 and on 5.7.1

WP 4.9.8	WP 5.7.1

Also, the Validated URL and Error Index counts show values on version 4.9.8 (which wasn't the case before).