Problem

There is no stable API for setting window-local extmarks.

Relevant links:

• Window-local display of extmarks #19654 - initial issue for window-local extmarks.
• feat(extmark): window scoped namespace #27361 - initial implementation of features that also make window-local extmarks possible.
• refactor(api): nvim_win_xx_ns are EXPERIMENTAL #28432 - marking the initial implementation as experimental.
• feat: set properties on namespaces #28728 - introduction of more general and experimental nvim__ns_{set,get}() functions.

Expected behavior

Functions nvim_ns_get() and nvim_ns_set() are not private and not experimental.

This is a feature request and not a PR because in this comment @justinmk mentioned that there needs something to be written up. This issue may serve as a place to do that.