checker: `as` type test

vlib/v/checker/tests/unknown_as_type.out

@@ -0,0 +1,7 @@
+x.v:7:9: error: unknown type `Stringg`
+    5 |
+    6 | fn foo(e Expr) {
+    7 |     x := e as Stringg
+      |            ~~
+    8 |     println(x)
+    9 | }

vlib/v/checker/tests/unknown_as_type.vv

@@ -0,0 +1,13 @@
+type Expr = Int | String
+
+struct Int {}
+struct String {}
+
+fn foo(e Expr) {
+	x := e as Stringg
+	println(x)
+}
+
+fn main() {
+
+}