feature: metadata extension works for single file

[gordonwoodhull]

Continuation of

• Brand extension #13193
• brand extension: find project dir #13221

This extends singleFileProjectContext to ingest metadata extensions.

This completely works and includes tests to prove it (with an extension but without _quarto.yml).

However, I was concerned that it has unknown side effects:

• It structures the output directories differently.
• I think I saw it change the output directory (but maybe I was mistaken).
• What else does it do? We don't know!

It is a clear generalization from what we currently have:

1. singleFileProjectContext takes RenderOptions instead of RenderFlags.
2. ExportmergeExtensionMetadata from project-context.ts.
3. Create an empty config.project. (It could have type: default, which should do the same thing.)
4. mergeExtensionMetadata into the project.

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