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/method/newlines.wren

@@ -1,7 +1,7 @@
 class Foo {
   construct new() {}
-  method(a, b) { "method " + a + " " + b }
-  [a, b] { "subscript " + a + " " + b }
+  method(a, b) { "method %(a) %(b)" }
+  [a, b] { "subscript %(a) %(b)" }
 }
 
 var foo = Foo.new()