Problem:
The nvim_win_xx_ns function family introduced in ba0370b1d7
needs more bake-time. Currently it's narrowly defined for windows, but
other scopes ("buffer") and features are likely in the future.
Solution:
- Rename the API with double-underscore to mark it as EXPERIMENTAL.
TODO/FUTURE:
- Rename and change the signature to support more than just "window"
scope, and for other flexibility.
- Open question: we could choose either:
- "store scopes on namespaces", or
- "store namespaces on scopes (w:/b:/…)"
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