fix(api): return "style" in nvim_win_get_config() #38122

Problem: nvim_win_get_config() does not return a window's "style".

Solution: always include it, and document `style=""`.

Always included so it can be used reciprocally with nvim_open_win() or
nvim_win_set_config(). (otherwise the config of a window with kWinStyleUnused
will not unset the kWinStyleMinimal style of another window if passed to
nvim_win_set_config, for example)

runtime/lua/vim/_meta/api_keysets.lua

@@ -462,7 +462,7 @@ error('Cannot require a meta file')
 --- @field mouse? boolean
 --- @field relative? "cursor"|"editor"|"laststatus"|"mouse"|"tabline"|"win"
 --- @field row? number
---- @field style? "minimal"
+--- @field style? ""|"minimal"
 --- @field noautocmd? boolean
 --- @field vertical? boolean
 --- @field win? integer