patch 8.1.1734: the evalfunc.c file is too big

Problem:    The evalfunc.c file is too big.
Solution:   Move some functions to other files.
diff --git a/src/json.c b/src/json.c
index 3a5de70..c9a6230 100644
--- a/src/json.c
+++ b/src/json.c
@@ -1127,3 +1127,52 @@
     return ret;
 }
 #endif
+
+/*
+ * "js_decode()" function
+ */
+    void
+f_js_decode(typval_T *argvars, typval_T *rettv)
+{
+    js_read_T	reader;
+
+    reader.js_buf = tv_get_string(&argvars[0]);
+    reader.js_fill = NULL;
+    reader.js_used = 0;
+    if (json_decode_all(&reader, rettv, JSON_JS) != OK)
+	emsg(_(e_invarg));
+}
+
+/*
+ * "js_encode()" function
+ */
+    void
+f_js_encode(typval_T *argvars, typval_T *rettv)
+{
+    rettv->v_type = VAR_STRING;
+    rettv->vval.v_string = json_encode(&argvars[0], JSON_JS);
+}
+
+/*
+ * "json_decode()" function
+ */
+    void
+f_json_decode(typval_T *argvars, typval_T *rettv)
+{
+    js_read_T	reader;
+
+    reader.js_buf = tv_get_string(&argvars[0]);
+    reader.js_fill = NULL;
+    reader.js_used = 0;
+    json_decode_all(&reader, rettv, 0);
+}
+
+/*
+ * "json_encode()" function
+ */
+    void
+f_json_encode(typval_T *argvars, typval_T *rettv)
+{
+    rettv->v_type = VAR_STRING;
+    rettv->vval.v_string = json_encode(&argvars[0], 0);
+}