HighlightingAssets: Extract new public API method get_syntax_for_path()

[Enselic]

We do this mainly to reduce duplication of logic. The move to adjust the public API is more of a side effect. It so happens that adjusting the public API also simplifies the code.

One of the reasons this refactoring works is that the .file_name() of a file_name is file_name unchanged:

    #[test]
    fn file_name_of_file_name_is_same() {
        let path = Path::new("test.rs");
        let expected = Path::new("test.rs");
        assert_eq!(path.file_name().unwrap_or_default(), expected);
    }