Add tracks event for the "add new post" button post-publish

[p-jackson]

Changes proposed in this Pull Request

• Adds tracking to the "Add new post/page" button

This button was added in WordPress/gutenberg#33276 and we want to know how much it gets used.

p1628731010423300-slack-C029SB8JT8S

Read more about testing tracks events in the block editor: PCYsg-nrf-p2

Testing instructions

• Sandbox widgets.wp.com
• Run in your sandbox: install-plugin.sh wpcom-block-editor add/tracks-new-post-button-after-publish
• Open network tools and filter by wpcom_block_editor_post_publish_add_new_click
• Ensure the network tools are persisting logs (part of testing involves a page transition, so we don't want the event to disappear before we can verify it)
• Publish a post/page
• Check that the wpcom_block_editor_post_publish_add_new_click event was sent and that the post_type property is set to either 'post' or 'page'

Because this tracking depends on a CSS selector I'll add e2e test to we're notified if the selector breaks. I'll add the test in a follow up PR because the tests will just fail until this change has merged (tests are run against the production version of widgets.wp.com) #55483

[github-actions]

Link to Calypso live: https://calypso.live?image=registry.a8c.com/calypso/app:build-12797
Link to Jetpack Cloud live: https://calypso.live?image=registry.a8c.com/calypso/app:build-12797&env=jetpack

[matticbot]

This PR modifies the release build for wpcom-block-editor

To test your changes on WordPress.com, run install-plugin.sh wpcom-block-editor add/tracks-new-post-button-after-publish on your sandbox.

To deploy your changes after merging, see the documentation: PCYsg-l4k-p2

[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.

[arthur791004]

LGTM 👍