feat(api): allow open_win/win_set_buf in the cmdwin in some cases

Problem: As discussed on Matrix, there was some interest in having
`nvim_open_win` again be able to open floats in the cmdwin (e.g: displaying a
hover doc related to what's in the cmdwin). After #23228, this was disallowed.

Solution: Allow `nvim_open_win` in the cmdwin as long as `!enter` and
`buffer != curbuf` (the former can cause all sorts of issues, and the latter
can crash Nvim after closing cmdwin). Also allow `nvim_win_set_buf` in a similar
fashion.

Note that we're not *entirely* sure if this is 100% safe (cmdwin is a
global-state-using-main-loop-calling beast), but this seems to work OK..?

Also:
  - Check the buffer argument of `nvim_open_win` earlier, and abort if it's
    invalid (it used to still open a window in this case).

  - Untranslate `e_cmdwin` errors in the API (other errors in the API are not
    translated: although not detailed in the API contract yet, errors are
    supposed to be stable).

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