fix: keep heading type on first line when splitting after Shift+Enter

[BetterAndBetterII]

Summary

• Lexical HeadingNode.insertNewAfter treats any text offset === 0 as the start of the heading, so Enter after a Shift+Enter linebreak moves the heading onto the latter fragment and downgrades the first line to a paragraph.
• Override insertNewAfter in headingsPlugin so a split after a linebreak keeps the first half as a heading, inserts a paragraph for the second half, and drops the leftover break. Start/end Enter behavior is unchanged.

Fixes #925

Test plan

• npx vitest --run src/test/headings.test.tsx (split after linebreak, Enter at start, Enter at end)
• npx vitest --run src/test/markdown-shortcut.test.tsx src/test/core.test.tsx

[petyosi]

Thank you for investigating this and for the detailed tests. Since the same behavior reproduces in the official Lexical Playground, the fix belongs in Lexical's HeadingNode.insertNewAfter implementation. I don't want MDXEditor to override HeadingNode.prototype globally as a workaround. Please submit the fix and regression test upstream to Lexical. Once the fix is merged and released there, I will upgrade MDXEditor's Lexical dependencies to include it.