Initial implementation of markdownlint-micromark package, micromark.mjs helpers, and tests.

test/every-markdown-syntax.md

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+Every Markdown Syntax
+=====================
+
+## Level 2 ATX Heading
+
+### Level 3 Closed ATX Heading ###
+
+---
+
+Text *emphasized* **strong** ___emphasized+strong___.
+Text `code` <strike>html</strike> <https://example.com/page>.
+Text [link](https://example.com/page) [link][] [link] ![image][link].
+
+Hard  
+line break
+
+[link]: https://example.com/page "Title"
+
+> Block quote
+> > Nested
+
+- Unordered
+- List
+  - Items
+    Indented
+
+    Content
+
+1. Ordered
+2. List
+   1. Items
+      Indented
+
+      Content
+
+```markdown options
+Fenced code block
+```
+
+    Indented code block
+
+<p>
+HTML block
+</p>
+
+<!-- markdownlint-disable-file -->