feat: abstract List's toString into generic join method on Sequence

Add `join` and `join(sep)` methods to the `Sequence` class, allowing any sequence to produce a concatenated string of its elements with an optional separator. Refactor `List.toString` to delegate to `join(", ")` with bracket wrapping, removing the manual iteration logic from List. Include new test files for List, Range, and String join behavior, plus runtime error tests for non-string separators.

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.