Make sure top level package can always be determined

The default `linkcode_resolve()` function parses the package's top level module if it's not provided as a confval.

If a custom `linkcode_resolve()` is used and `top_level` isn't set, won't be able to determine the top level module

Author: TDKorn

docs/source/README.rst

@@ -137,7 +137,7 @@ Syntax Highlighting
 
    .. literalinclude:: ../../sphinx_github_style/__init__.py
       :language: python
-      :lines: 226-238
+      :lines: 232-244
 
 .. only:: readme or pypi
 

sphinx_github_style/__init__.py

@@ -33,7 +33,13 @@ def setup(app: Sphinx) -> Dict[str, Any]:
     top_level = get_conf_val(app, 'top_level')
     TDKLexer.TOP_LEVEL = top_level
 
-    if not callable(linkcode_func):
+    if callable(linkcode_func):
+        if not top_level:
+            raise ExtensionError(
+                "``sphinx-github-style``: must set the `top_level` confval"
+                " or use the default `linkcode_resolve` function"
+            )
+    else:
         print(
             "Function `linkcode_resolve` not found in ``conf.py``; "
             "using default function from ``sphinx_github_style``"