"Browse" tab should always return to the browser UI, not the detail page

This patch creates a distinct project_browser.detail route for the project detail pages. It has the unanticipated side effect of hiding the "Extend" local task group entirely while looking at the detail pages. This was initially off-putting, but I came to believe I was looking for the tabs only because I had been working on them specifically. With the tabs gone, the "Back to Browsing" link is much more prominent and useful and is no longer competing with the tabs for an end user's attention.

The approach is also consistent with other navigations in Drupal Core. For example, the Block Layout page is inside of a local tasks tabset, but the detail pages for configuring individual blocks are not.

It is tempting to move the module title into the Drupal page title after this change, but that would be functionally complicated because the existing plumbing is built to expose project data to Svelte through an API and not for direct use by PHP code. Moving the title may also lead to a cascade of other design changes.

I think this might be better served by making the detail page a modal and having this problem go away entirely. See https://www.drupal.org/project/project_browser/issues/3310908 ✨ Consider making 'detail page' a modal window instead of a separate "page" in app Active