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/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
 System.print("a")
 import "shared" for Shared
-var A = "a " + Shared
+var A = "a %(Shared)"
 System.print("a done")

test/language/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
 System.print("b")
 import "shared" for Shared
-var B = "b " + Shared
+var B = "b %(Shared)"
 System.print("b done")