docs: add mapColumns examples

[psh4607]

작업 배경

Issue #475 asks for concrete documentation of both supported mapColumns forms, especially the callback that receives an Electric ChangeMessage.

티켓 및 링크

• GH-475 — Closes pglite-sync: document mapColumns option, especially the callback version #475

작업 내용

• Mark mapColumns as optional, matching the current SyncShapeToTableOptions API.
• Add an object-form example that maps local table columns to shape columns.
• Add a callback-form example that reads ChangeMessage.value and returns values keyed by local column names.

테스트

• pnpm --dir docs stylecheck
• pnpm --dir docs typecheck
• VitePress Markdown renderer structural check for the option list and both TypeScript code blocks
• git diff --check
• pnpm --dir docs build — blocked before page rendering because this fresh worktree does not contain the generated packages/pglite/dist/index.js workspace artifact