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/__tests__/pathUtils.test.ts

@@ -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 {isSamePath} from '../pathUtils';
+
+describe('isSamePath', () => {
+  test('should be true for compared path without trailing slash', () => {
+    expect(isSamePath('/docs', '/docs')).toBeTruthy();
+  });
+
+  test('should be true for compared path with trailing slash', () => {
+    expect(isSamePath('/docs', '/docs/')).toBeTruthy();
+  });
+
+  test('should be false for compared path with double trailing slash', () => {
+    expect(isSamePath('/docs', '/docs//')).toBeFalsy();
+  });
+
+  test('should be true for twice undefined/null', () => {
+    expect(isSamePath(undefined, undefined)).toBeTruthy();
+    expect(isSamePath(undefined, undefined)).toBeTruthy();
+  });
+
+  test('should be false when one undefined', () => {
+    expect(isSamePath('/docs', undefined)).toBeFalsy();
+    expect(isSamePath(undefined, '/docs')).toBeFalsy();
+  });
+});