feat(v2): @docusaurus/theme-common (#3775)

* create base @docusaurus/theme-common package + fix Webpack client export aliases issue shadowing other theme-common package

* Move theme-classic/src/utils code to new @docusaurus/theme-common package

* add prettierignore

* fix bad test location for getDocusaurusAliases()

packages/docusaurus-theme-common/src/utils/docsPreferredVersion/useDocsPreferredVersion.ts

@@ -0,0 +1,66 @@
+/**
+ * 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 {useCallback} from 'react';
+import {useDocsPreferredVersionContext} from './DocsPreferredVersionProvider';
+import {useAllDocsData, useDocsData} from '@theme/hooks/useDocs';
+
+import {DEFAULT_PLUGIN_ID} from '@docusaurus/constants';
+
+// TODO improve typing
+
+// Note, the preferredVersion attribute will always be null before mount
+export function useDocsPreferredVersion(
+  pluginId: string | undefined = DEFAULT_PLUGIN_ID,
+) {
+  const docsData = useDocsData(pluginId);
+  const [state, api] = useDocsPreferredVersionContext();
+
+  const {preferredVersionName} = state[pluginId];
+
+  const preferredVersion = preferredVersionName
+    ? docsData.versions.find(
+        (version: any) => version.name === preferredVersionName,
+      )
+    : null;
+
+  const savePreferredVersionName = useCallback(
+    (versionName: string) => {
+      api.savePreferredVersion(pluginId, versionName);
+    },
+    [api],
+  );
+
+  return {preferredVersion, savePreferredVersionName} as const;
+}
+
+export function useDocsPreferredVersionByPluginId() {
+  const allDocsData = useAllDocsData();
+  const [state] = useDocsPreferredVersionContext();
+
+  function getPluginIdPreferredVersion(pluginId: string) {
+    const docsData = allDocsData[pluginId];
+    const {preferredVersionName} = state[pluginId];
+
+    return preferredVersionName
+      ? docsData.versions.find(
+          (version: any) => version.name === preferredVersionName,
+        )
+      : null;
+  }
+
+  const pluginIds = Object.keys(allDocsData);
+
+  const result: Record<
+    string,
+    any // TODO find a way to type this properly!
+  > = {};
+  pluginIds.forEach((pluginId) => {
+    result[pluginId] = getPluginIdPreferredVersion(pluginId);
+  });
+
+  return result;
+}