Add a proper test for multiple interfaces

Without inspecting the resulting WAT, and instead concluding based on
the test compiling successfully and executing without any errors,
everything seems to work properly.

Author: CountBleck

tests/compiler/multiple-interfaces.debug.wat

@@ -0,0 +1 @@
+{}

tests/compiler/multiple-interfaces.js

@@ -0,0 +1,63 @@
+interface W {
+  w: f64
+}
+
+interface X {
+  x: f64
+}
+
+interface Y {
+  y: f64
+}
+
+interface Z {
+  z: f64
+}
+
+// WX XY XZ YZ
+// |  \__|__/
+// |     |
+// |    Vec3
+// \_____|
+//      Vec4
+
+interface WX extends W, X {}
+interface XY extends X, Y {}
+interface XZ extends X, Z {}
+interface YZ extends Y, Z {}
+
+interface Vec3 extends XY, XZ, YZ {}
+
+/**
+ * Who needs a regular old Vec3, when you can have a Vec4?
+ * Vec3 is overused anyway :^)
+ */
+interface Vec4 extends WX, Vec3 {}
+
+class Vec4TheClass implements Vec4 {
+  constructor(
+    public w: f64,
+    public x: f64,
+    public y: f64,
+    public z: f64
+  ) {}
+}
+
+const vec4 = new Vec4TheClass(1, 2, 3, 4);
+
+const anotherVec4: Vec4 = vec4;
+assert(anotherVec4.w == vec4.w);
+
+const y: Y = vec4;
+assert(y.y == vec4.y);
+
+const yz: YZ = vec4;
+assert(yz.z == vec4.z);
+
+const vec3: Vec3 = vec4;
+const x: X = vec3;
+assert(vec3.x == x.x);
+
+// Up-cast, down-cast, left-cast, right-cast
+const wx = vec3 as Vec4 as WX;
+assert(wx.w == vec4.w);