feat(core): backport 4 improvements to serializer from prosemirror-markdown upstream

[d3m1d0v]

Backports 4 improvements from the prosemirror-markdown upstream into the fork's MarkdownSerializerState:

• Non-strict render fallback — serialize() with strict: false no longer throws on unknown node types; instead, block nodes render their content and leaf nodes are silently skipped
• getMark() with strict fallback — unknown mark types throw in strict mode (default) and are silently ignored in non-strict mode, instead of causing a runtime error
• Smarter expelEnclosingWhitespace — trailing whitespace is no longer expelled from a mark when that mark continues on the next sibling node (fixes spurious spaces between adjacent same-mark nodes)
• atBlockStart tracking — start-of-line escape logic now uses a dedicated atBlockStart flag instead of the atBlank() || closed heuristic, making #, -, > etc. escape only at actual block start, not after inline line breaks

Tests:

• Ported relevant test cases from prosemirror-markdown/test1.ts to Markdown.test.ts
• Added new tests covering non-strict mode for unknown nodes and marks, expelEnclosingWhitespace with mark continuation, and atBlockStart precision for start-of-line escaping

[gravity-ui]

[gravity-ui]