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.

src/wren_core.c

@@ -85,6 +85,20 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
+"  join { join(\"\") }\n"
+"\n"
+"  join(sep) {\n"
+"    var first = true\n"
+"    var result = \"\"\n"
+"\n"
+"    for (element in this) {\n"
+"      if (!first) result = result + sep\n"
+"      first = false\n"
+"      result = result + element.toString\n"
+"    }\n"
+"\n"
+"    return result\n"
+"  }\n"
 "}\n"
 "\n"
 "class String is Sequence {}\n"
@@ -97,15 +111,7 @@ static const char* libSource =
 "    return other\n"
 "  }\n"
 "\n"
-"  toString {\n"
-"    var result = \"[\"\n"
-"    for (i in 0...count) {\n"
-"      if (i > 0) result = result + \", \"\n"
-"      result = result + this[i].toString\n"
-"    }\n"
-"    result = result + \"]\"\n"
-"    return result\n"
-"  }\n"
+"  toString { \"[\" + join(\", \") + \"]\" }\n"
 "\n"
 "  +(other) {\n"
 "    var result = this[0..-1]\n"