improved support for inline-font-files(), adds font-formats() method

this modifies `inline-font-files` so it can take the same syntax as `font-files`
```scss
// old syntax (still works)
$files: inline-font-files('path/to/file.ttf', truetype);

// new syntax (without the format specified)
$files: inline-font-files('path/to/file.ttf');
```

this also fixes an issue where `font-files` couldn't take an unquoted string (because of the regex pattern used):
```scss
$files: font-files( unquote('path/to/file.ttf') );
```

and added a `font-formats` method that will return a list of formats given the above syntaxes:
```scss
$formats: font-formats('path/to/file.ttf'); // truetype
$formats: font-formats('path/to/file.ext', 'otf'); // opentype
$formats: font-formats('path/to/file.ttf', 'path/to/file.ext', 'otf'); // (truetype, opentype)
```

each of these methods (`font-files`, `inline-font-files`, `font-formats`) each uses shared logic for deriving the format `with_each_font_file`