patch 8.2.2806: Vim9: using "++nr" as a command might not work

Problem:    Vim9: using "++nr" as a command might not work.
Solution:   Do not recognize "++" and "--" in a following line as addition or
            subtraction.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 0d87a89..85fecbb 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -3531,6 +3531,13 @@
 	    eap->cmdidx = CMD_eval;
 	    return eap->cmd;
 	}
+
+	// Check for "++nr" and "--nr".
+	if (p == eap->cmd && p[0] == p[1] && (*p == '+' || *p == '-'))
+	{
+	    eap->cmdidx = *p == '+' ? CMD_increment : CMD_decrement;
+	    return eap->cmd + 2;
+	}
     }
 #endif