…ys sanitized following the font collection schema

- Sanitize and validate as early as possible, so that appropriate notices are logged right away
- Use static method for sanitization schema so that we can use a closure rather than a public method for src sanitization
- Adds a check for WP_Font_Utils::sanitize_from_schema so that class callables like array( $this, 'sanitization_method'	) can be used
- Updates method name to indicate that sanitization is done first, as this might remove invalid data and affect the validation result