chore: add acceptance tests for if/is

examples/acceptance_tests/105/lib/tests.ak

@@ -0,0 +1,45 @@
+pub type Foo {
+  a: Int,
+}
+
+pub type Bar {
+  Bazz(Int)
+  Buzz(Int)
+}
+
+test if_soft_cast() {
+  let d: Data = Foo { a: 1 }
+
+  if d is Foo {
+    d.a == 1
+  } else {
+    False
+  }
+}
+
+test if_soft_cast_2() {
+  let d: Data = Bazz(1)
+
+  if d is Foo {
+    d.a == 1
+  } else if d is Bazz(y): Bar {
+    y == 1
+  } else {
+    False
+  }
+}
+
+test if_soft_cast_3() {
+  let d: Data = Bazz(1)
+  let x: Data = Buzz(2)
+
+  if d is Foo {
+    d.a == 1
+  } else if d is Bazz(y): Bar {
+    y == 1
+  } else if x is Buzz(y): Bar {
+    y == 2
+  } else {
+    False
+  }
+}