Fix Appearance > Template Parts URL

[Mamaduka]

Due to recent changes to Site Editor's client-side routing, the Appearance > Template Parts URL requires an extra argument to load the page correctly.

This a ticket for syncing the fix with Core: ​https://github.com/WordPress/gutenberg/pull/48343.

Testing instructions

1. Activate a theme that enables block-based template parts - ​https://github.com/Mamaduka/block-fragments.
2. Visit the new Appearance > Template Parts menu.
3. Confirm the list page is loaded correctly.

The flow is now also covered by e2e tests in the Gutenberg plugin repo.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/48343

Trac ticket: https://core.trac.wordpress.org/ticket/57819

[hellofromTonya]

Currently reviewing for commit ahead of Beta 4 today.

[hellofromTonya]

Testing Instructions

These steps define how to reproduce the issue, and indicate the expected behavior.

Steps to Reproduce

1. Install the block-fragments theme (i.e. enables block-based template parts within a non-block theme) ​https://github.com/Mamaduka/block-fragments.
2. Activate the theme.
3. Navigate to Appearance > Template Parts menu.
4. 🐞 Bug occurs.

Expected Results

When testing a patch to validate it works as expected:

• ✅ The Site Editor should show All template parts in the menu sidebar and a list of template parts.

When reproducing a bug:

• ❌ The Site Editor shows the Design menu sidebar with an empty content area.

[hellofromTonya]

Test Report: Can reproduce the issue (before applying PR 4142)

[hellofromTonya]

Test Report: PR 4142 resolves the issue

[hellofromTonya]

Test Report

Patch tested: ​https://github.com/WordPress/wordpress-develop/pull/4142/

Environment

• OS: macOS
• Localhost: wp-env (Docker)
• WordPress: trunk
• Browser: Firefox, Chrome, Edge
• Theme: Block Fragments
• Plugins: none

Actual Results

When reproducing a bug/defect:

• ❌ Can reproduce the reported issue.

When testing the bugfix patch:

• ✅ The patch resolves the issue.

[hellofromTonya]

In 55435:

Site Editor: Add 'path' query arg to Appearance > Template Parts menu.

To display a list of block-based template parts for classic themes, this changeset adds a path query arg to the Appearance > Template Parts menu URL. This change is necessary due to recent changes to Site Editor's client-side routing.

Reference:

• ​Gutenberg PR 48343
• ​Gutenberg PR 42729

Follow-up to [55392], [54176].

Props mamaduka, audrasjb, fabiankaegy, hellofromTonya, kevin940726, mukesh27, youknowriad.
Fixes #57819.

Committed via https://core.trac.wordpress.org/changeset/55435.