patch 8.2.0355: Vim9: str_val is confusing, it's a number

Problem:    Vim9: str_val is confusing, it's a number
Solution:   Rename to stnr_val.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index c9f72f5..6618b6a 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -773,8 +773,8 @@
     RETURN_OK_IF_SKIP(cctx);
     if ((isn = generate_instr(cctx, ISN_STORENR)) == NULL)
 	return FAIL;
-    isn->isn_arg.storenr.str_idx = idx;
-    isn->isn_arg.storenr.str_val = value;
+    isn->isn_arg.storenr.stnr_idx = idx;
+    isn->isn_arg.storenr.stnr_val = value;
 
     return OK;
 }
@@ -3789,8 +3789,8 @@
 		    garray_T	*stack = &cctx->ctx_type_stack;
 
 		    isn->isn_type = ISN_STORENR;
-		    isn->isn_arg.storenr.str_idx = idx;
-		    isn->isn_arg.storenr.str_val = val;
+		    isn->isn_arg.storenr.stnr_idx = idx;
+		    isn->isn_arg.storenr.stnr_val = val;
 		    if (stack->ga_len > 0)
 			--stack->ga_len;
 		}