String "cat-1" is a valid date

**Describe the bug**
`dayjs('cat-1').isValid()` returns `true`

**Expected behavior**
`dayjs('cat-1').isValid()` should return `false`

**Information**
- Day.js Version v1.8.19
- OS: MacOS Catalina
- Browser: Chrome Version 79.0.3945.88 (Official Build) (64-bit)
- Time zone: /

**UPDATE**
After a fast analysis, I found that Dayjs uses `Date` inside `parseDate` method and the problem is really inside `Date` indeed `new Date("cat-1")` returns `2000-12-31T23:00:00.000Z`.
After that, I think that Dayjs should manage this case.