Motivation / Background

This Pull Request has been created because as mentioned in #54573, the output of bin/rails routes doesn't filter engine routes properly.

Closes #54573

Detail

This Pull Request changes the routing inspect to do the following:

• Always unfurl engine routes so that all routes of an application can be filtered as needed
• Improve heading, messaging, and formatting of the inspector to denote app routes, when engines have no routes, spacing between engines, etc.
• Only adds additional formatting on applications using engines

Additional information

Since we rewrote how routes are collected and printed, I did a quick time benchmark on a large app to make sure we're not slowing things down:

Before:

bin/rails routes  4.73s user 3.26s system 84% cpu 9.513 total
bin/rails routes -g filter  4.75s user 3.14s system 90% cpu 8.766 total

After:

bin/rails routes  4.44s user 2.68s system 75% cpu 9.431 total
bin/rails routes -g filter  4.88s user 3.19s system 90% cpu 8.889 total

Same-ish to a bit faster on a full print. Applying a filter is slightly slower, but that's expected because it iterates through all routes (app and engine) now instead of lazy unfurling that previously missed embedded routes.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.