refactor: looser fenced code block handling

Fixes #602

.changeset/chilled-kings-retire.md

@@ -0,0 +1,5 @@
+---
+'markdown-to-jsx': patch
+---
+
+Fenced code blocks are now tolerant to a missing closing sequence; this improves use in LLM scenarios where the code block markdown is being streamed into the editor in chunks.

index.compiler.spec.tsx

@@ -3809,6 +3809,18 @@ Yeah boi
       </pre>
     `)
   })
+
+  it('regression 602 - should treat anything following ``` as code until the closing pair', () => {
+    render(compiler('```\nfoo'))
+
+    expect(root.innerHTML).toMatchInlineSnapshot(`
+      <pre>
+        <code>
+          foo
+        </code>
+      </pre>
+    `)
+  })
 })
 
 describe('indented code blocks', () => {

index.tsx

@@ -183,7 +183,7 @@ const BLOCKQUOTE_TRIM_LEFT_MULTILINE_R = /^ *> ?/gm
 const BREAK_LINE_R = /^ {2,}\n/
 const BREAK_THEMATIC_R = /^(?:( *[-*_])){3,} *(?:\n *)+\n/
 const CODE_BLOCK_FENCED_R =
-  /^\s*(`{3,}|~{3,}) *(\S+)?([^\n]*?)?\n([\s\S]+?)\s*\1 *(?:\n *)*\n?/
+  /^(?: {1,3})?(`{3,}|~{3,}) *(\S+)? *([^\n]*?)?\n([\s\S]*?)(?:\1\n?|$)/
 const CODE_BLOCK_R = /^(?: {4}[^\n]+\n*)+(?:\n *)+\n?/
 const CODE_INLINE_R = /^(`+)\s*([\s\S]*?[^`])\s*\1(?!`)/
 const CONSECUTIVE_NEWLINE_R = /^(?:\n *)*\n/