Add support for language-specific formatting

If a language tag is present, we currently won't end up doing anything, i.e this test will fail:

#[test]
fn language_specific_formatting() -> Result {
  Test::new()?
    .markdown(
      r#"
      ```rust present echo "fn main() { println!(\"Hello, World!\"); }"
      ```
      "#,
    )
    .expected_status(0)
    .expected_stdout(
      r#"
      ```rust present echo "fn main() { println!(\"Hello, World!\"); }"
      fn main() { println!("Hello, World!"); }
      ```
      "#,
    )
    .run()
}

We can make it pass by looking for a subset of keywords as language tags, and then doing the entire parse -> command interpolation flow.

Start with the language_specific_formatting test shown in the issue and run it to confirm the current failure. Trace the markdown parsing and command interpolation flow, then make recognized language tags follow the full parse-to-interpolation path; done means the test passes with the expected formatted output.