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/highlight_defs.h

@@ -160,6 +160,8 @@ typedef enum {
   kHlSyntax,
   kHlTerminal,
   kHlCombine,
+  kHlBlend,
+  kHlBlendThrough,
 } HlKind;
 
 typedef struct {