Abstract List's toString method to a more general join method on Sequence.

test/range/join.wren

@@ -0,0 +1,4 @@
+var a = 1..3
+
+IO.print(a.join) // expect: 123
+IO.print(a.join(", ")) // expect: 1, 2, 3

test/range/join_sep_non_string.wren

@@ -0,0 +1 @@
+(1..3).join(2) // expect runtime error: Right operand must be a string.