Add exports in package.json

[tushuhei]

This PR improves the package distribution of Budoux to enhance compatibility and usability in various JavaScript environments.

Specifically, it addresses the following:

• Node.js ESM Support: Previously, the build process for ESM would incorrectly overwrite win.ts with win-browser.ts. This prevented Node.js environments from using the ESM build, as win-browser.ts relies on browser-specific APIs. This change ensures that Node.js can properly import and use the ESM version of Budoux.
• Enhanced Package Resolution with exports Field: The addition of the exports field in package.json provides explicit mappings for different module formats (CommonJS and ESM). This improves interoperability with modern JavaScript bundlers and frameworks, ensuring they correctly resolve the appropriate module format and avoid potential conflicts. This also simplifies usage for developers by providing clear entry points.

[viiccwen]

Hello, @tushuhei. Does this PR try to fix issue #888 ?

[tushuhei]

Partially. This should fix the first problem I pointed out.

[kojiishi]

LGTM