refactor(theme-common): split package into public/internal API entrypoints (#7660)

* add theme-common/internal export

* Split @docusaurus/theme-common into public/internal apis

* fixes

* public <-> private

* public <-> private

* public <-> private

* fix

* add "removeThemeInternalReexport" CI script

* :s windows CI check not working: not that useful

* remove bad import

* refactors

* reorder

* make useBackToTopButton internal

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

packages/docusaurus-theme-common/removeThemeInternalReexport.mjs

@@ -0,0 +1,32 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import fs from 'fs-extra';
+
+// See comment in src/internal.ts
+// This script should be run by CI tests to remove:
+// export * from './index'
+
+const filePath = 'lib/internal.js';
+const lineToRemove = "export * from './index';\n";
+
+if (!(await fs.pathExists(filePath))) {
+  throw new Error(`internal entrypoint file not found at ${filePath}`);
+}
+
+const fileContent = await fs.readFile(filePath, 'utf8');
+
+const fileContentUpdated = fileContent.replaceAll(lineToRemove, '');
+
+// Ensure the script correctly removes the re-export
+if (fileContent === fileContentUpdated) {
+  throw new Error(
+    'Unexpected: internal re-export has not been replaced.\nMake sure this script works, and is only run once.',
+  );
+}
+
+await fs.writeFile(filePath, fileContentUpdated);