Return the extension properties for constructing Duration from numbers,
but this time place them in Duration.Companion rather than on top-level.
- provide the new set of construction extension properties
in companion of Duration
- leave top-level extension properties experimental,
increase their deprecation level to ERROR,
propose the extension properties in companion instead.
- leave Duration companion static factory functions experimental,
deprecate them and propose the extension properties in companion instead.
Ilya Gorbunov
21a9198fff