feat(api): add nvim_win_resize()

Ref #6645

Problem:
When a window is resized it takes space from the window right/below first,
and only falls back to the window left/above when there is no more room.
Sometimes a user wants the space to come from a specific direction.

Solution:
Add nvim_win_resize(win, width, height, {anchor}) which resizes a window
with a choosable anchor edge, letting a window grow leftwards or upwards
by taking space from the window to the left or above first. The default
anchor reproduces nvim_win_set_width()/nvim_win_set_height().

runtime/lua/vim/lsp/completion.lua

@@ -677,7 +677,7 @@ local function update_popup_window(winid, bufnr, kind)
       vim.treesitter.start(bufnr, kind)
     end
     local all = api.nvim_win_text_height(winid, {}).all
-    api.nvim_win_set_height(winid, all)
+    api.nvim_win_resize(winid, -1, all, {})
   end
 end