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/_meta/api_keysets.gen.lua

@@ -493,6 +493,9 @@ error('Cannot require a meta file')
 --- @field zindex? integer
 --- @field _cmdline_offset? integer
 
+--- @class vim.api.keyset.win_resize
+--- @field anchor? string
+
 --- @class vim.api.keyset.win_text_height
 --- @field end_row? integer
 --- @field end_vcol? integer