Add rsx_file! macro for external RSX template files

[thefiddler]

Closes #148

This MR adds two new macros, rsx_file! and rsx_file_borrow! mirroring rsx! and rsx_borrow!, that allow users to split html code into separate .html files if they wish. This enables formatting and linting support for free using the default IDE language servers e.g. in Zed or VSCode.

Design choices:

• rsx_file! naming scheme for consistency with sqlxquery_file!
• Template path is relative to CARGO_MANIFEST_DIR. Unlike askama, I opted to not force an implicit templates/ directory with askama.toml override, but rather let the user choose the complete path. But happy to change if we want to increase consistency with askama.
• I use include_bytes! for dependency tracking so Cargo automatically recompiles when the template file changes
• Basic validation that the path is not absolute
• Basic testing

Example

  // templates/success.html
  <h1>"Success!"</h1>
  <p>"The operation completed successfully."</p>

  // src/lib.rs
  use hypertext::rsx_file;

  pub fn success_template() -> impl Renderable {
      rsx_file!("templates/success.html")
  }

Let me know your thoughts!

[thefiddler]

Closing in favour of #185