fix(richtext-lexical): link markdown regex should not match similar looking image markdown (#15713)

The current regex matches the link markdown correctly:

```
[This is a link](https://example.com/foo/bar)
```

But it also matches the image markdown:
```
![This is an image](https://example.com/foo/image.jpg)
```

To fix this, a negative lookbehind is added which confirms that no `!`
is present before the match.

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---------

Co-authored-by: German Jablonski <43938777+GermanJablo@users.noreply.github.com>

Author: shobhitsinghal624

packages/richtext-lexical/src/features/link/markdownTransformer.ts

@@ -29,8 +29,8 @@ export const LinkMarkdownTransformer: TextMatchTransformer = {
 
     return linkContent
   },
-  importRegExp: /\[([^[]+)\]\(([^()\s]+)(?:\s"((?:[^"]*\\")*[^"]*)"\s*)?\)/,
-  regExp: /\[([^[]+)\]\(([^()\s]+)(?:\s"((?:[^"]*\\")*[^"]*)"\s*)?\)$/,
+  importRegExp: /(?<!!)\[([^[]+)\]\(([^()\s]+)(?:\s"((?:[^"]*\\")*[^"]*)"\s*)?\)/,
+  regExp: /(?<!!)\[([^[]+)\]\(([^()\s]+)(?:\s"((?:[^"]*\\")*[^"]*)"\s*)?\)$/,
   replace: (textNode, match) => {
     const [, linkText, linkUrl] = match
     const linkNode = $createLinkNode({

test/lexical-mdx/int.spec.ts

@@ -248,6 +248,17 @@ describe('Lexical MDX', () => {
       expect(text).toMatch(/alt|image\.jpg|\/uploads\//)
     })
   })
+
+  describe('link markdown: should not match image markdown', () => {
+    it('should not parse image markdown as a link node', () => {
+      const markdown = '![Alt text](https://example.com/image.jpg)'
+      const result = mdxToEditorJSON({ mdxWithFrontmatter: markdown, editorConfig })
+      const serialized = JSON.stringify(result.editorState)
+      expect(serialized).toContain('"text":"![Alt text](https://example.com/image.jpg)"')
+
+      expect(serialized).not.toContain('"type":"link"')
+    })
+  })
 })
 
 function removeUndefinedAndIDRecursively(obj: object) {