feat(v2): various markdown string parsing improvements/fixes (#4590)

* extract createExcerpt code in separate file + add bad test

* almost working markdown parsing refactor

* complete parseMarkdownString refactor

* fix tests

* fix blog test issue

* fix docusaurus utils imports

packages/docusaurus-plugin-content-blog/src/blogFrontMatter.ts

@@ -0,0 +1,43 @@
+/**
+ * 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 {Joi} from '@docusaurus/utils-validation';
+import {Tag} from './types';
+
+// TODO complete this frontmatter + add unit tests
+type BlogPostFrontMatter = {
+  id?: string;
+  title?: string;
+  description?: string;
+  tags?: (string | Tag)[];
+  slug?: string;
+  draft?: boolean;
+  date?: string;
+};
+
+const BlogTagSchema = Joi.alternatives().try(
+  Joi.string().required(),
+  Joi.object<Tag>({
+    label: Joi.string().required(),
+    permalink: Joi.string().required(),
+  }),
+);
+
+const BlogFrontMatterSchema = Joi.object<BlogPostFrontMatter>({
+  id: Joi.string(),
+  title: Joi.string(),
+  description: Joi.string(),
+  tags: Joi.array().items(BlogTagSchema),
+  slug: Joi.string(),
+  draft: Joi.boolean(),
+}).unknown();
+
+export function assertBlogPostFrontMatter(
+  frontMatter: Record<string, unknown>,
+): asserts frontMatter is BlogPostFrontMatter {
+  Joi.attempt(frontMatter, BlogFrontMatterSchema);
+}