[tmm]

Description

The Rust port had no markdown shortcuts: typing # , - or **bold**
left the markdown as text. It also had no KEY_ENTER_COMMAND, which the
shortcuts' Enter handler listens on.

• markdown::register_markdown_shortcuts ports MarkdownShortcuts.ts.
  An update listener runs the element, multiline element, text match and
  text format transformers as markdown is typed. A KEY_ENTER_COMMAND
  handler runs the code fence and the element transformers that trigger
  on Enter. It honours the collaboration, historic and composition-end
  tags, the typed-one-character check ($getOffsetInParent) and
  HISTORY_PUSH_TAG.
• KEY_ENTER_COMMAND takes a KeyboardEvent (modifier keys and
  preventDefault), or None for JS's null. Its JS handlers are
  ported: registerRichText's (with $escapeFormatsForTrigger on the
  default triggers, and the block-decorator NodeSelection case),
  registerPlainText's, and CodeExtension's $exitCodeNodeOnEnter
  handler in register_code.
• ops.ts gets a markdown script flag
  (registerMarkdownShortcuts(editor, TRANSFORMERS)) and
  KEY_ENTER_COMMAND. The Rust and Swift script runners read both.
• scripts/markdown-shortcuts (15 cases) is recorded from JS. It covers
  headings, quotes, lists, code fences on space and on Enter, inline
  formats and links, what cancels them, several characters at once, the
  composition-end, collaboration and historic tags, and undo. It also adds
  one KEY_ENTER_COMMAND case each to scripts/rich-text-commands,
  scripts/plain-text and scripts/code, leaving their existing cases
  unchanged.

Swift matched JS on every new case, so no Swift source changes.

Test plan

Before

$ cargo run -p lexical-conformance -- scripts/
| `scripts/markdown-shortcuts` | – | – | – | – | – | not implemented |

After

$ cargo run -p lexical-conformance
| `scripts/code` | 10 | 10 | 0 | 0 | 0 | pass |
| `scripts/markdown-shortcuts` | 15 | 15 | 0 | 0 | 0 | pass |
| `scripts/plain-text` | 5 | 5 | 0 | 0 | 0 | pass |
| `scripts/rich-text-commands` | 7 | 7 | 0 | 0 | 0 | pass |
| **Total** | 655 | 655 | 0 | 0 | 0 | |

$ swift test --filter ScriptTests   # Linux, Swift 6.4.2
✔ Suite ScriptTests passed

$ pnpm exec vitest run src/scripts.test.ts   # packages/lexical-conformance
      Tests  19 passed (19)

cargo fmt --all --check, cargo clippy --workspace --all-targets --all-features --locked -- -D warnings, cargo test --workspace --all-features --locked, RUSTDOCFLAGS='-D warnings' cargo doc --workspace --all-features --no-deps --locked and
cargo +1.88 check -p lexical-editor --all-features --locked pass.