Summary

This PR removes the Prism-driven coupling between markdown and code packages by doing two coordinated changes:

1. Extract Prism highlighting ownership from @lexical/code into @lexical/code-prism.
2. Make markdown code-fence support explicit composition (instead of implicit default behavior in @lexical/markdown).

Context

A previous attempt (#8196) proposed a lightweight @lexical/code/node subpath to avoid importing the Prism-heavy @lexical/code root from markdown internals.

Based on maintainer feedback (comment), this PR follows the structural direction instead:

• move Prism out to a dedicated package,
• avoid introducing an interim awkward subpath.

Problem

@lexical/markdown and @lexical/code were coupled such that optional code/highlighting paths affected markdown-only use cases. This made modular composition weaker than intended.

Changes

1) Prism extraction

• Added @lexical/code-prism package.
• Moved Prism-specific runtime/highlighting logic there.
• Updated internal consumers (playground/integration fixtures) to import Prism APIs from @lexical/code-prism.

2) @lexical/code slim-down

• @lexical/code keeps code-node/core behavior.
• Prism ownership is no longer in @lexical/code root.

3) Markdown composition change

• @lexical/markdown no longer has a direct runtime dependency on @lexical/code.
• TRANSFORMERS no longer includes code-fence behavior by default.
• Added createMarkdownCodeBlockTransformer(...) so code-fence behavior can be composed explicitly by consumers.
• Added multiline transformer isNode predicate support so shortcut skip behavior remains correct for composed node types.

4) Docs/tests updates

• Updated markdown docs and React plugin docs to show explicit code-fence composition.
• Added/updated unit tests for:
  • default core transformers (no code-fence -> CodeNode),
  • explicit composed code-fence behavior.

Semver-visible behavior change

@lexical/markdown default TRANSFORMERS is now code-agnostic.
Consumers that want fenced code block conversion to CodeNode must compose it explicitly.

Migration

import {
  TRANSFORMERS,
  createMarkdownCodeBlockTransformer,
} from '@lexical/markdown';
import {$createCodeNode, $isCodeNode, CodeNode} from '@lexical/code';

const CODE = createMarkdownCodeBlockTransformer({
  $createCodeNode,
  $isCodeNode,
  dependencies: [CodeNode],
});

const transformers = [...TRANSFORMERS, CODE];

Validation

• pnpm exec vitest run packages/lexical-markdown/src/__tests__/unit/LexicalMarkdown.test.ts packages/lexical-markdown/src/__tests__/unit/MarkdownTransformers.test.ts
• pnpm exec vitest run packages/lexical-markdown/src/__tests__/unit/LexicalMarkdownCodeComposition.test.ts
• pnpm exec vitest run scripts/__tests__/unit/build.test.ts
• pnpm run flow
• pnpm run tsc

Issue links

Fixes #5381
Related to #4550 #5424 #6575