Add css_file_path helper to LeptosOptions (see #3513)

[Alex-Barbosu]

Context

Users need a convenient way to get the path to the generated CSS file without manually reconstructing it.

Problem

LeptosOptions did not provide a built-in method for retrieving the CSS path, so users had to duplicate logic.

Solution

Added a new method css_file_path() to LeptosOptions that returns the correct CSS file path as a string.

Example

let leptos_options = LeptosOptions {
    site_pkg_dir: "pkg".into(),
    output_name: "app".into(),
    ..Default::default()
};

println!("CSS path: {}", leptos_options.css_file_path());