[patrick-immometrica]

Closes #1044

Adds Tera::get_referenced_templates(name). It returns the names of the templates a template references through extends and include, plus the ones those templates reference in turn. It sits next to get_template_variables and works the same way.

I went with the full set of templates instead of only the direct references that jinja2.meta.find_referenced_templates returns. For dependency tracking you want to know that page.html depends on nav.html when base.html is the template that includes it. Tera already resolves the parent chain, so this is a short walk over parents and include_calls.

Differences from the Jinja function:

• No None entries. Tera only accepts string literals in extends and include, so every name is known.
• No imports, because Tera 2 has none.
• Names come back after fallback prefixes are resolved, so you get the name the template was added under.
• The template itself is not part of the result.

Like get_template_variables, it reports every reference it finds, including ones inside an if or inside a block that a child overrides. It also reports the parent of an included template that uses extends. I'd rather over-report than have a site generator skip a rebuild it needed.

I added a test in tests/introspection.rs that covers an extends chain with includes at each level, nested includes, a fallback prefix, an included template that extends another one, a template with no references, and an unknown template name.