These changes adds an `exports` field to package.json with the main
goal of allowing the root/main export to be our ESM syntax version of
the package, as opposed to the UMD/CommonJS version.

This will make it easier for using projects written with ESM to use
mustache.js out of the box, without having to figure out that they'd
have to `import mustache/mustache.mjs` to get the ESM version.

It is also assumed to be beneficial for tools like http://skypack.dev
to choose the correct ESM version when appropriate.

Refs https://nodejs.org/api/packages.html#packages_package_entry_points

This is a precursor to introducing a build step that will change what
we expose from this package. Better off writing some tests to verify
existing projects with different module systems continue to work as
expected.