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/module/cyclic_import/b.wren

@@ -2,8 +2,8 @@
 System.print("start b")
 
 var B = "b value"
-System.print("b defined " + B)
+System.print("b defined %(B)")
 import "a" for A
-System.print("b imported " + A)
+System.print("b imported %(A)")
 
 System.print("end b")