fix(api): make open_win block only enter/leave events if !enter && !noautocmd

Problem: nvim_open_win blocking all win_set_buf autocommands when !enter &&
!noautocmd is too aggressive.

Solution: temporarily block WinEnter/Leave and BufEnter/Leave events when
setting the buffer. Delegate the firing of BufWinEnter back to win_set_buf,
which also has the advantage of keeping the timing consistent (e.g: before the
epilogue in enter_buffer, which also handles restoring the cursor position if
autocommands didn't change it, among other things). Reword the documentation for
noautocmd a bit.

I pondered modifying do_buffer and callees to allow for BufEnter/Leave being
conditionally disabled, but it seems too invasive (and potentially error-prone,
especially if new code paths to BufEnter/Leave are added in the future).

Unfortunately, doing this has the drawback of blocking ALL such events for the
duration, which also means blocking unrelated such events; like if window
switching occurs in a ++nested autocmd fired by win_set_buf. If this turns out
to be a problem in practice, a different solution specialized for nvim_open_win
could be considered. :-)

test/functional/api/window_spec.lua

@@ -1620,6 +1620,40 @@ describe('API/win', function()
       )
       command('new | quit')
     end)
+
+    it('restores last known cursor position if BufWinEnter did not move it', function()
+      -- This test mostly exists to ensure BufWinEnter is executed before enter_buffer's epilogue.
+      local buf = api.nvim_get_current_buf()
+      insert([[
+        foo
+        bar baz .etc
+        i love autocommand bugs!
+        supercalifragilisticexpialidocious
+        marvim is actually a human
+        llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch
+      ]])
+      api.nvim_win_set_cursor(0, { 5, 2 })
+      command('set nostartofline | enew')
+      local new_win = api.nvim_open_win(buf, false, { split = 'left' })
+      eq({ 5, 2 }, api.nvim_win_get_cursor(new_win))
+
+      exec([[
+        only!
+        autocmd BufWinEnter * ++once normal! j6l
+      ]])
+      new_win = api.nvim_open_win(buf, false, { split = 'left' })
+      eq({ 2, 6 }, api.nvim_win_get_cursor(new_win))
+    end)
+
+    it('does not block all win_set_buf autocommands if !enter and !noautocmd', function()
+      local new_buf = fn.bufadd('foobarbaz')
+      exec([[
+        let triggered = ""
+        autocmd BufReadCmd * ++once let triggered = bufname()
+      ]])
+      api.nvim_open_win(new_buf, false, { split = 'left' })
+      eq('foobarbaz', eval('triggered'))
+    end)
   end)
 
   describe('set_config', function()