chore: release 0.4.0

[luojiyin1987]

Release 0.4.0

Prepare the npm release. This is a minor bump: 0.3.2 → 0.4.0.

Reason for the minor bump

v0.3.2..HEAD adds one public API. New public API is a semver minor change.

Changes in this PR

File	Change
CHANGELOG.md	Move the Unreleased entry into a new 0.4.0 section.
package.json	Set version to 0.4.0.

Unreleased content

• Add MarkdownSourceMap.getCodeSourceInfo() (source map: expose code block structure info (fenced vs indented, openingFence, infoInsertPoint) #141, feat: expose code block structure info from the source map #142).

Validation

• pnpm run test: 25 suites, 2017 tests pass.
• pnpm run test:types: type declarations compile.
• pnpm run test:package: publint passes. The packed tarball CJS/ESM smoke test passes.
• etc/parser.api.md is unchanged.

After merge

Tag v0.4.0 and push the tag. The tag starts the release workflow. Then publish to npm.