I believe `string` is a library type, not a keyword.
You could add it to [the `primary` key](https://github.com/geany/geany/blob/0a0b4b000cf18e837b921057a461ad05a08e41d3...). Alternatively, you could add it to [the `types` key](https://github.com/geany/geany/blob/0a0b4b000cf18e837b921057a461ad05a08e41d3...) if you wanted to highlight it different from built-in types and keywords.