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/a.wren

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