Add dual CommonJS/ESM and Deno support

[zuchka]

Summary

• preserve the existing CommonJS implementation and add a thin native ESM entry point
• add conditional exports with matching CommonJS and ESM TypeScript declarations
• preserve established deep paths and package metadata/document paths when introducing the export map
• support Deno 2 through the npm package
• document CommonJS, Node ESM, and Deno usage and add an unreleased 0.7.0 changelog entry

This replaces and supersedes #116 while retaining credit for the original ESM/Deno proposal from @tukkek. The existing index.js implementation is unchanged.

Performance issue #118 is intentionally separate from this PR.

Compatibility coverage

• existing behavior suite: 39 passing tests
• packed-tarball CommonJS, ESM, legacy deep-path, metadata, README, and license consumers
• Node.js 18, 20, 22, 24, and 26 package tests
• TypeScript CommonJS/ESM consumer compilation plus Are the Types Wrong validation
• deterministic differential comparison against published remove-markdown@0.6.4 across 11,165 input/option combinations
• esbuild and Rollup browser-targeted consumer bundles
• Deno 2 source and packed npm-package checks
• package smoke tests on Linux, macOS, and Windows in CI
• reproducible npm ci, zero audit findings, and clean workflow lint

Rollout gates

• Let the draft CI matrix complete on GitHub
• Publish the exact candidate as a prerelease under a non-latest dist-tag
• Run representative downstream application suites against the exact prerelease
• Hold a short prerelease soak window and review install/runtime reports
• Promote to 0.7.0 only after those gates are green

No package has been published as part of this PR.