extension navbar merging needs order

Bug description

When a custom project type defined by an extension configures navbar entries, a project which uses that extension cannot control the order that its own navbar entries are merged.

For example, when the extension has configuration like:

contributes:
  project:
    project:
      type: website
    website:
      navbar:
        pinned: true
        right:
         - icon: "list"
           menu:
           - text: "docs.posit.co"
             href: "https://docs.posit.co"
           - text: "Posit Support"
             href: "https://support.posit.co/hc/en-us/"

A project using this extension might have additional navbar configuration like:

website:
  navbar:
    right:
      - text: "Release Notes"
        url: "https://docs.posit.co/ide/news/"

When rendered, the navbar entries from the project-type are rendered to the left of the navbar entries from the project itself.

Custom project types and projects should have some way of determining the final order for navbar items.

This same problem likely happens when other types of list-like configurations are merged, like page-footer entries.

Steps to reproduce

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Expected behavior

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Actual behavior

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Your environment

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Quarto check output

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Start by tracing how navbar entries from extension-defined project types are merged with entries from the project configuration. Reproduce the example using the shown YAML and compare the rendered order, then determine how list-like settings such as page-footer entries are handled. Done means projects can explicitly control the final order of merged navbar items.