json: fix encode/decode of fixed arrays (fix #22369) (#22448)

vlib/json/json_fixed_array_test.v

@@ -0,0 +1,25 @@
+import json
+
+struct Base {
+	options  Options
+	profiles Profiles
+}
+
+struct Options {
+	cfg [6]u8
+}
+
+struct Profiles {
+	cfg [4][7]u8
+}
+
+fn test_main() {
+	a := json.encode(Base{})
+	println(a)
+	assert a.contains('"cfg":[[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0]]')
+
+	b := json.decode(Base, a)!
+	assert b.options.cfg.len == 6
+	assert b.profiles.cfg.len == 4
+	assert b.profiles.cfg[0].len == 7
+}