Execute input in the REPL.

Guesses whether the input is an expression or statement and handles it
appropriately. Finally, after over a year, the Wren REPL automatically
prints "3" if you type in "1 + 2". \o/

test/language/variable/newline_after_equals.wren

@@ -0,0 +1,5 @@
+var foo =
+
+
+    123
+System.print(foo) // expect: 123