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/io/stdin/read_line.wren

@@ -1,9 +1,9 @@
 import "io" for Stdin
 
 System.write("> ")
-System.print("1 " + Stdin.readLine())
+System.print("1 %(Stdin.readLine())")
 System.write("> ")
-System.print("2 " + Stdin.readLine())
+System.print("2 %(Stdin.readLine())")
 
 // stdin: first
 // stdin: second