Start working on string escapes.

test/string_escapes.wren

@@ -0,0 +1,8 @@
+// Escape characters.
+io.write("\"")     // expect: "
+io.write("\\")     // expect: \
+io.write("(\n)")   // expect: (
+                   // expect: )
+
+// TODO(bob): Non-printing escapes like \t.
+// TODO(bob): Unicode escape sequences.

test/string_literals.wren

@@ -1,4 +1,2 @@
 io.write("".count)   // expect: 0
 io.write("a string") // expect: a string
-
-// TODO(bob): Escapes.