Display a different sign when there's a breakpoint on the PC line

Vim only renders a single sign-per-line. If we have the PC _and_ a
breakpoint, we should make that clear. Do this using a vimspectorPCBP
sign which combines both vimspectorPC and vimspectorBP (sort of).

We can't (unfortuantely) render the breakpoint blob in a different
colour, but it's at least obvious when we toggle on the PC line.

tests/lib/autoload/vimspector/test/signs.vim

@@ -32,7 +32,7 @@ function! vimspector#test#signs#AssertPCIsAtLineInBuffer( buffer, line ) abort
   let index = 0
   while index < len( signs[ 0 ].signs )
     let s = signs[ 0 ].signs[ index ]
-    if s.name ==# 'vimspectorPC'
+    if s.name ==# 'vimspectorPC' || s.name ==# 'vimspectorPCBP'
       if assert_false( pc_index >= 0, 'Too many PC signs' )
         return 1
       endif