patch 9.0.1227: no cmdline completion for :runtime

Problem:    No cmdline completion for :runtime.
Solution:   Add completion for :runtime. (closes #11853, closes #11447)
            Improve the resulting matches.
diff --git a/src/usercmd.c b/src/usercmd.c
index 3bd6fd5..9dcc20e 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -86,6 +86,7 @@
 #endif
     {EXPAND_SETTINGS, "option"},
     {EXPAND_PACKADD, "packadd"},
+    {EXPAND_RUNTIME, "runtime"},
     {EXPAND_SHELLCMD, "shellcmd"},
 #if defined(FEAT_SIGNS)
     {EXPAND_SIGN, "sign"},