Generate typesVersions entry in package.json automatically

[chrisguttandin]

This is a spin-off of the discussion started in #23. Theoretically it would be possible for this tool to modify the typesVersions entry of a package.json file automatically depending on the features used.

If for example no feature is used which requires any down-leveling no typesVersions property could be created at all.

If for example a private field is used in the codebase downlevel-dts could create a folder with the down-leveled files and at the same time it could update the typesVersions entry to point to that folder.

If for example a private field and accessors are used in the same codebase downlevel-dts could create two folders with the down-leveled files for TypeScript below 3.8 and for TypeScript below 3.7. It could also update the typesVersions entry to point to those two folders.