feat: automatically strip YAML front matter (#538)

Co-authored-by: Evan Jacobs <570070+quantizor@users.noreply.github.com>

fixture.md

@@ -1,3 +1,7 @@
+---
+draft: false
+date: 2024-01-15
+---
 Markdown: Syntax
 ================
 

index.compiler.spec.tsx

@@ -4043,6 +4043,24 @@ describe('overrides', () => {
   })
 })
 
+it('should remove YAML front matter', () => {
+  render(
+    compiler(theredoc`
+      --- 
+      key: value
+      other_key: different value
+      --- 
+      Hello.
+    `)
+  )
+
+  expect(root.innerHTML).toMatchInlineSnapshot(`
+    <span>
+      Hello.
+    </span>
+`)
+})
+
 it('handles a holistic example', () => {
   const md = fs.readFileSync(__dirname + '/fixture.md', 'utf8')
   render(compiler(md))

index.tsx

@@ -190,6 +190,7 @@ const CR_NEWLINE_R = /\r\n?/g
 const FOOTNOTE_R = /^\[\^([^\]]+)](:.*)\n/
 const FOOTNOTE_REFERENCE_R = /^\[\^([^\]]+)]/
 const FORMFEED_R = /\f/g
+const FRONT_MATTER_R = /^---[ \t]*\n(.|\n)*\n---[ \t]*\n/
 const GFM_TASK_R = /^\s*?\[(x|\s)\]/
 const HEADING_R = /^ *(#{1,6}) *([^\n]+?)(?: +#*)?(?:\n *)*(?:\n|$)/
 const HEADING_ATX_COMPLIANT_R =
@@ -1078,6 +1079,8 @@ export function compiler(
   }
 
   function compile(input: string): JSX.Element {
+    input = input.replace(FRONT_MATTER_R, '')
+
     let inline = false
 
     if (options.forceInline) {