Extend Optimizer with transformer for module/nomodule scripts (ESM)

[westonruter]

Feature description

See ampproject/amp-toolbox#722.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[schlessera]

This work should also include preloading of critical modules, either within the same transformer or a separate one.

See https://developers.google.com/web/updates/2017/12/modulepreload

[schlessera]

The draft PR ampproject/amp-wp#5078 needs to be transferred over to this repository.

[kristoferbaxter]

Please note that the AMP Performance WG does not yet recommend the usage of modulepreload, but instead uses a correctly configured preload for the Google AMP Cache.

Would prefer this change does not use modulepreload and mirrors the functionality on the Google AMP Cache when module mode is enabled.

[westonruter]

Please note that the AMP Performance WG does not yet recommend the usage of modulepreload, but instead uses a correctly configured preload for the Google AMP Cache.

Note that modulepreload has been implemented as of ampproject/amp-toolbox#1161, but apparently only temporarily so.

[westonruter]

@sebastianbenz Should we be going live in WP with preload or modulepreload?

[kristoferbaxter]

There was a recent regression in preload when used with module scripts in Chromium.

Engineers on the V8 team are working to address, and as a temporary measure we've elected to use modulepreload on the Google AMP Cache. This carries a downside, as Safari will not process these directives.

Until this is resolved, modulepreload is the best of the bad options.