[tmm]

Description

The Rust port's html feature had a DOM (lexical_editor::dom) but no
node rendered into it, so html/export and html/import had no suite.

This adds, behind the same feature:

• createDOM, exportDOM and the static importDOM as LexicalNode
  hooks, with ElementNode.exportDOM (indent and direction) and
  TextNode's behavior as their defaults, and the types they exchange
  (DOMExportOutput, DOMConversionOutput, DOMConversionMap, ...) in
  lexical_editor::lexical_dom, ported from LexicalNode.ts,
  LexicalEditor.ts and the DOM helpers of LexicalUtils.ts.
• createEditor's theme (always) and html config (export overrides
  and extra import conversions), editor._config, the conversion cache,
  and DEFAULT_EDITOR_DOM_CONFIG.
• The DOM code of the core nodes: TextNode (createDOM with the theme's
  class names and the style, exportDOM, and the #text, b, span and
  format-element conversions), TabNode, LineBreakNode,
  ParagraphNode (with its extractWithChild), and
  ArtificialNode__DO_NOT_USE, which the core now registers as JS does.
• LexicalNode.isSelected.
• lexical_editor::html, ported from packages/lexical-html/src/index.ts:
  generate_html_from_nodes, generate_dom_from_nodes,
  generate_dom_from_root, append_node_to_html (with
  $sliceSelectedTextNodeContent for a selection) and
  generate_nodes_from_dom.
• Conformance suites for html/export and html/import.

$inlineStylesFromStyleSheets and the DOMImportExtension and
DOMRenderExtension pipelines are not ported (rust/PARITY.md); no vector
needs them. The node packages' DOM parts come next.

Test plan

Before

$ cargo run -q -p lexical-conformance --release -- html/
| `html/entities` | 2125 | 2125 | 0 | 0 | 0 | pass |
| `html/export` | – | – | – | – | – | not implemented |
| `html/import` | – | – | – | – | – | not implemented |
| `html/parse` | 37 | 37 | 0 | 0 | 0 | pass |
| **Total** | 2162 | 2162 | 0 | 0 | 0 | |

After

$ cargo run -q -p lexical-conformance --release -- html/
| `html/entities` | 2125 | 2125 | 0 | 0 | 0 | pass |
| `html/export` | 42 | 42 | 0 | 0 | 0 | pass |
| `html/import` | 68 | 68 | 0 | 0 | 0 | pass |
| `html/parse` | 37 | 37 | 0 | 0 | 0 | pass |
| **Total** | 2272 | 2272 | 0 | 0 | 0 | |

$ cargo run -q -p lexical-conformance --release
| **Total** | 3643 | 3643 | 0 | 0 | 0 | |

cargo fmt --all --check, cargo clippy --workspace --all-targets --all-features --locked -- -D warnings, cargo test --workspace --all-features --locked, cargo doc with -D warnings, and
cargo check -p lexical-editor --no-default-features --features <f> for
each feature are clean.