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

test/string/join.wren

@@ -0,0 +1,5 @@
+var str = "string"
+
+IO.print(str.join("") == str) // expect: true
+
+IO.print(str.join(", ")) // expect: s, t, r, i, n, g

test/string/join_sep_non_string.wren

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