fix: make wit_library world attribute optional for interface libraries

WASI interface packages (wasi:io, wasi:cli, etc.) export interfaces but don't
define worlds. The world attribute is only needed for component entry points,
not for interface library definitions.

This fixes CI build failures where WASI wit_library targets were missing the
mandatory world attribute.

Author: avrabe

wit/wit_library.bzl

@@ -204,8 +204,8 @@ wit_library = rule(
             doc = "WIT package name (defaults to target name)",
         ),
         "world": attr.string(
-            mandatory = True,
-            doc = "World name defined in the WIT file (required for predictable binding generation)",
+            mandatory = False,
+            doc = "World name defined in the WIT file (optional, typically only needed for component entry points)",
         ),
         "interfaces": attr.string_list(
             doc = "List of interface names defined in this library",