runtime(termdebug): allow multibyte characters as breakpoint signs (#14274)

Allow multibyte characters as termdebug signs using slice() function

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Mihai Ciuraru <mihai.ciuraru@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim b/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
index 1dce91b..50833f0 100644
--- a/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
+++ b/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
@@ -1661,7 +1661,7 @@
       endif
     endif
     call sign_define('debugBreakpoint' .. nr,
-				\ #{text: strpart(label, 0, 2), 
+				\ #{text: slice(label, 0, 2),
 				\ texthl: hiName})
   endif
 endfunc