feat: add the plugin system to the project

[ramboz]

Follow up of #254 that now leverages aem.js.

Requires https://github.com/adobe/aem-lib/pull/23

Test URLs:

• Before: https://main--aem-boilerplate--adobe.hlx.live/
• After: https://plugin-system--aem-boilerplate--ramboz.hlx.live/

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[ramboz]

@rofe and they are 😉 https://github.com/adobe/aem-lib/pull/23 The changes here are a copy/paste of the aem-lib build from that branch

[rofe]

Ok, then I suggest to let's park this PR until https://github.com/adobe/aem-lib/pull/23 is approved and merged.

[ramboz]

@rofe ok moving it back to a draft PR then