Clarify separate types for CJS and ESM in package.json redirect strategy#6

The [package-json-redirect README](https://github.com/andrewbranch/example-subpath-exports-ts-compat/tree/main/examples/node_modules/package-json-redirects) describes separate types for CJS and ESM files also being possible with the package.json redirect strategy. > Note that the separate handling of ESM and CJS, as well as the decision to present a unified set of types for the two, is orthogonal to the subpath exports demonstration. (Including separate types for CJS and ESM files, as [extensionless](https://github.com/andrewbranch/example-subpath-exports-ts-compat/blob/main/examples/node_modules/extensionless) does, is often a better choice, and is compatible with this strategy.) How is this compatible, with just one types field of the package.json would that be accomplished with two sets of types in a redirected-to directory with .d.mts and .d.ts extensions? And the types field should have no extension? When I type it out it seems obvious, I guess my confusion was around which file extensions tricks were unique to the "extensionless" approach.