Add missing support for subpaths

[hl662]

When using export and subpaths, to fully support CJS and ESM, we need each subpath to contain import and require.

Previously, ./Renderer didn't support CJS, because it didn't have a require field. This would cause problems in packages, like itwinjs/core core-full-stack-tests, that consumes both Main and Renderer, but outputs CJS files. At build time, this wouldn't be caught, but during runtime, it would throw a PACKAGE_PATH_NOT_SUPPORTED, because when it tried to use the Renderer subpath, it couldn't find a valid CJS path.

Vice versa, with ./Main in ESM outputted files.

This issue is fixed by adding both import and require and their valid values to each subpath.

[ben-polinsky]

Excellent. I ran into this setting up my docs cospace the other day. Thanks!