Upgrade js-yaml to v5.1.0 and migrate to Vitest from Jest

[Copilot]

Bumps js-yaml to the new v5 line and confirms markdown-it meets the ≥14.2.0 requirement.

Changes

• js-yaml ^5.1.0 added as a direct devDependency, hoisting js-yaml@5.1.0 to the root of node_modules (was 4.1.1 via transitive hoisting from @eslint/eslintrc and markdownlint-cli2)
• markdown-it already pinned at 14.2.0 as a direct dependency — no change required

Remaining nested older versions

These transitive consumers still resolve older nested copies. All are at their latest published versions; no upgrade path exists without npm overrides.

Package	Nested dep
markdownlint-cli2@0.22.1	markdown-it@14.1.1
@eslint/eslintrc@3.3.5 (via eslint)	js-yaml@4.2.0
@istanbuljs/load-nyc-config@1.1.0 (via jest → @jest/transform → babel-plugin-istanbul)	js-yaml@3.14.2

[JoyceZhu]

@copilot, can you try migrating Jest to Vitest to get rid of the @istanbuljs/load-nyc-config dependency?

[Copilot]

Pull request overview

This PR updates the repository’s Node dependency graph to ensure js-yaml@5.1.0 is installed at the workspace root (as a direct devDependency) and refreshes the lockfile accordingly, while confirming markdown-it already satisfies the stated version requirement.

Changes:

• Add js-yaml ^5.1.0 as a direct devDependency in package.json.
• Update package-lock.json to reflect js-yaml@5.1.0 at the root and corresponding dependency tree adjustments.
• Update the lockfile’s recorded root engines.node range (as part of the lockfile update).

Show a summary per file

File	Description
package.json	Adds js-yaml as a direct dev dependency to pull in v5.x at the root.
package-lock.json	Updates the resolved dependency tree for js-yaml and adjusts recorded root package metadata (including engines).

Copilot's findings

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• Files reviewed: 1/2 changed files
• Comments generated: 0

[Copilot]

@copilot, can you try migrating Jest to Vitest to get rid of the @istanbuljs/load-nyc-config dependency?

Done in the latest commit. Here's what changed:

• jest removed from devDependencies; vitest ^4.1.9 added in its place
• jest.config.json removed; vitest.config.js added (globals: true so test files need no import changes)
• Test script simplified from NODE_OPTIONS=--experimental-vm-modules jest → vitest run (Vitest handles ESM natively)
• overrides["@babel/core"] removed — it was only needed by Jest's Babel transform pipeline
• eslint.config.js updated: globals.jest → globals.vitest, and vitest added to the import/no-unresolved ignore list

All 21 tests pass and linting is clean.