Implement SPI to support search highlight

[cyanzhong]

Closes #87. When review, focus on logic correctness only, majority of the code is AI-generated.

[Copilot]

Pull request overview

Implements a preview-side Search SPI so the editor can drive search highlighting and match navigation inside the rendered Markdown preview (closes #87).

Changes:

• Add src/search.ts implementing mark/unmark, match indexing, and preview-side counter info.
• Expose the Search SPI on window.__markeditPreviewSPI__ for CoreEditor to call.
• Add unit tests for the preview search behavior using happy-dom, and add mark.js as a dependency.

Reviewed changes

Copilot reviewed 4 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/search.ts	New search/highlighting engine for the preview using mark.js, plus counter/index helpers.
main.ts	Exposes performSearch, setSearchMatchIndex, clearSearch, searchCounterInfo over a global SPI.
types/global.d.ts	Types the global SPI on window for TS consumers.
types/markjs.d.ts	Adds minimal mark.js type declarations.
tests/search.test.ts	Adds tests covering preview-mode counter behavior, search, index wrapping, and clearing.
package.json	Adds runtime dependency mark.js and dev dependency happy-dom for tests.
tsconfig.json	Suppresses TS deprecation warnings via ignoreDeprecations.
yarn.lock	Lockfile updates for newly added dependencies.

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