Feature Request - dayjs.safeParse() for safe parsing, especially for strings

My idea is,

```typescript
function safeParse (s: any): Dayjs {
if (typeof s === 'string') {
let m: RegExpExecArray | null = null

if (typeof s !== 'string') {
throw new Error(`Not a string: ${s} -- ${typeof s}`)
}

if (m = /(\d{4})(\d{2})(\d{2})?/.exec(s)) {
return dayjs(`${m[1].padStart(2, '0')}-${m[2].padStart(2, '0')}-${(m[3] || '1').padStart(2, '0')}`)
}

if (/[-+]?(?:\d+(?:\.\d+)?|\.\d+)/.test(s)) {
throw new Error(`Is an invalid Number string: ${s}`)
}

return dayjs(s)
}
...
}
```

Actually, I have already implemented this, with testing, here. -- https://github.com/patarapolw/q2object/tree/master/packages/is-datestring

```
Is a valid Number string
✓ '20190101'

✓ '201902'

Is an invalid Number string
✓ '1'

✓ '0'

✓ '-1'

✓ '1.1'

✓ '0.1'

✓ '.1'

✓ '-.1'

Not a string
✓ 1

✓ true

✓ null

✓ ! ''

✓ ! 'function () { }'

✓ 2020-01-07T18:24:03.053Z
```