UI: implement 'pumblend' option for semi-transparent popupmenu

Why?

- Because we can.
- Because the TUI is just another GUI™
- Because it looks kinda nice, and provides useful context like 1 out of 100
  times

Complies with "don't pay for what you don't use".

Some crashes for resizing were unfolded, add tests for those.

src/nvim/options.lua

@@ -1805,6 +1805,14 @@ return {
       varname='p_ph',
       defaults={if_true={vi=0}}
     },
+    {
+      full_name='pumblend', abbreviation='pb',
+      type='number', scope={'global'},
+      vi_def=true,
+      redraw={'ui_option'},
+      varname='p_pb',
+      defaults={if_true={vi=0}}
+    },
     {
       full_name='pyxversion', abbreviation='pyx',
       type='number', scope={'global'},