fix: ensure date-fns#parse is called with a string

[lebalz]

Currently, the webuntis api of our school returns the school year as number. This creates an error when calling parse(20240403, 'yyyyMMdd', new Date()) (from date-fns).

The proposed fix ensures the parse function of date-fns is called always with a string by redefining the parse function (which is really only a simple wrapper that transforms dateStr to a string).:

import { parse as fnsParse, /*...*/ type ParseOptions } from 'date-fns';
// ...
const parse = <DateType extends Date>(
    dateStr: string | number,
    formatStr: string,
    referenceDate: DateType | number | string,
    options?: ParseOptions,
) => {
    return fnsParse(`${dateStr}`, formatStr, referenceDate, options);
};
// ...

Thanks for having a look and btw. i like your project! Thanks :)