Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/language/interpolation/switch_fiber.wren

@@ -0,0 +1,8 @@
+var fiber = Fiber.new {
+  System.print("in fiber")
+  Fiber.yield("result")
+}
+
+System.print("outer %(fiber.call()) string")
+// expect: in fiber
+// expect: outer result string