vim-patch:9.2.0645: Composing chars no longer accepted in end-id abbr

Problem:  Composing chars are no longer accepted in end-id abbreviation
          (after 9.2.0629).
Solution: Unescape all chars using vim_unescape_csi() instead of using
          mb_unescape() on individual chars, so that mb_ptr2len() and
          MB_PTR_ADV() can still be used.

closes: vim/vim#20514

23a84d28a8

test/old/testdir/test_mapping.vim

@@ -30,6 +30,18 @@ func Test_abbreviation()
   iunab abc；
   iunab ；
 
+  " abbreviation with composing chars (end-id)
+  inoreab ..ã a^~
+  inoreab ..β̃ β^~
+  inoreab ..π̃ π^~
+  inoreab ..Λ̃ Λ^~
+  call feedkeys("Go..ã ..β̃ ..π̃ ..Λ̃ \<Esc>", "xt")
+  call assert_equal('a^~ β^~ π^~ Λ^~ ', getline('$'))
+  iunab ..ã
+  iunab ..β̃
+  iunab ..π̃
+  iunab ..Λ̃
+
   bwipe!
 endfunc