Metrics: Add a new e2e/metrics folder to start writing some test to collect performance metrics

[youknowriad]

The ultimate goal is to have a wp.com or calypso dashboard in https://www.codevitals.run
This needs to be done in three steps (like we did before for woo, Gutenberg and core):

1- Add a script to be able to run and connect performance metrics on a given calypso instance.
2- Add a CI job to be able to run these metrics tests (and compare two different commits on that same run)
3- Log the results of that job result in codevitals.run

This PR attempts to address the first step here. It is based on this similar PR from Woo. woocommerce/woocommerce#42005 but it also uses the exact same setup as the calypso e2e tests (using these as a basis)

The PR is not ready yet because I want to add the "performance reporter" to the playwright config of the "metrics" test runner. The performance reporter class can be copy/pasted from the Woo PR but I'm having trouble finding the right way to add it to the playwright config. Help from someone familiar with calypso-e2e is needed.

For now I added a test that loads a large post on the post editor in Calypso and collects some basic loading metrics (the same one we use in woo and Gutenberg editor tests)

Testing instructions

cd test/e2e && yarn build && yarn metrics

[matticbot]

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[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.