Add exports field to support Node native ESM

Editor assessment

This is what a high-impact build commit should look like. `build` is the right type for adding the package.json `exports` field: no source logic changed, but the packaging change is visible enough to deserve prominence in the changelog. The subject 'add exports field to support Node native ESM' explains both the change and the motivation in nine words, is imperative, and fits length constraints easily. Anyone scanning history for Node ESM compatibility will find this instantly. No scope is a reasonable choice for a root-level packaging concern, though `(package)` would work. There is no body, and for a change this consequential a note on which entry points map to which export conditions would have been valuable. Still, excellent.