[@mantine/core] fix: `isNumberLike` should only check for valid css

[Kenzo-Wada]

fixes: https://github.com/mantinedev/mantine/issues/6853

updated isNumberLike func so that it'll return true when it's valid css props. otherwise false. eg) 2xl

I've checked all test passes, and added test for custom sizes

[rtivital]

Thanks!