de-de not supported

**Describe the bug**
dayjs does not ship with the locale de-de

**Expected behavior**
When reading the locale from navigator.language, the German locale in Germany (not Switzerland nor Austria) is set to `de-de`. When passed to dayjs, there is an error as `dayjs/locale/de-de.js` is not there

**Information**
- Day.js Version latest
- Browser chrome latest

Start by inspecting the locale files and the missing dayjs/locale/de-de.js entry, then reproduce the issue with navigator.language set to de-de. Done means Day.js accepts de-de without an error and uses the German locale for Germany rather than the Switzerland or Austria variants.

Written by the indexing model from the issue text.