Breaking changes in 1.10.4

**Describe the bug**
#1338 introduced breaking change by removing DurationInputType and DurationAddType types from plugin namespace.
(https://github.com/iamkun/dayjs/commit/4aca4b1b584a15de1146d929f95c944594032f20#diff-ca2786d2c104006cc51bdc06948ed0fad7843dba6f8ca1570eaedea389c9da88L8)

**Expected behavior**
No breaking changes within patch release (if following semantic versioning). Any plans to revert this change?

**Information**
- Day.js Version [e.g. v1.10.4]
- OS: [e.g. iOS]
- Browser [e.g. chrome 62]
- Time zone: [e.g. GMT-07:00 DST (Pacific Daylight Time)]

Start by reviewing commit 4aca4b1b584a15de1146d929f95c944594032f20 and the change referenced in its diff. Trace the plugin namespace definitions for DurationInputType and DurationAddType, then compare the public API before and after the change; done means the patch-release compatibility concern is addressed and the affected type behavior is verified.

Written by the indexing model from the issue text.