feat: move contains method from List to Sequence and add tests for list and range

Add a generic `contains(element)` method to the `Sequence` class that iterates over elements using the existing `for` loop protocol, removing the duplicate implementation from `List`. Include comprehensive test files for both `List.contains` and `Range.contains` covering ordered, backwards, and exclusive ranges. Also fix the `generate_builtins.py` script to write to the correct `src/vm/` directory path and add a `float:left` CSS rule to the main content area in the documentation stylesheet.

src/vm/wren_core.c

@@ -45,6 +45,15 @@
 // This string literal is generated automatically from core. Do not edit.
 static const char* libSource =
 "class Sequence {\n"
+"  contains(element) {\n"
+"    for (item in this) {\n"
+"      if (element == item) {\n"
+"        return true\n"
+"      }\n"
+"    }\n"
+"    return false\n"
+"  }\n"
+"\n"
 "  count {\n"
 "    var result = 0\n"
 "    for (element in this) {\n"
@@ -138,15 +147,6 @@ static const char* libSource =
 "    }\n"
 "    return result\n"
 "  }\n"
-"\n"
-"  contains(element) {\n"
-"    for (item in this) {\n"
-"      if (element == item) {\n"
-"        return true\n"
-"      }\n"
-"    }\n"
-"    return false\n"
-"  }\n"
 "}\n"
 "\n"
 "class Map {\n"