Editor: Preload template lookup

[Mamaduka]

What?

This is similar to #66654.

PR adds a post-type single template lookup to the REST API preloaded paths.

Why?

PR tries to optimize editor loading speed, especially when the default rendering mode is 'template-locked' and the editor has to wait for template resolution.

Testing Instructions

1. Open a page.
2. Inspect requests using the DevTools Network tab.
3. Confirm that there's only one template lookup request (this is a different issue, and I plan to resolve it separately).

Testing Instructions for Keyboard

Same.

Screenshots or screencast

[github-actions]

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[github-actions]

There was a conflict while trying to cherry-pick the commit to the wp/6.8 branch. Please resolve the conflict manually and create a PR to the wp/6.8 branch.

PRs to wp/6.8 are similar to PRs to trunk, but you should base your PR on the wp/6.8 branch instead of trunk.

# Checkout the wp/6.8 branch instead of trunk.
git checkout wp/6.8
# Create a new branch for your PR.
git checkout -b my-branch
# Cherry-pick the commit.
git cherry-pick fc030b3c883b9593ceefb42b893600bcfd723b82
# Check which files have conflicts.
git status
# Resolve the conflict...
# Add the resolved files to the staging area.
git status
git add .
git cherry-pick --continue
# Push the branch to the repository
git push origin my-branch
# Create a PR and set the base to the wp/6.8 branch.
# See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request.