Eliminate .NET version .js suffix in module import

[jasongin]

This fixes a small usability issue that has been bugging me while writing docs...

When loading a specific version of .NET using ES import, the .js suffix was required after the target framework moniker:

import dotnet from 'node-api-dotnet/net6.0.js'

While the .js suffix is normal for importing ES module files, the versions are not really meant to be modules but alternative entry-points. Anyway this uses the exports property in package.json to declare those entry-points without a .js suffix:

import dotnet from 'node-api-dotnet/net6.0'

(Package entry-points can be conditional and work with CommonJS also. I might use them in some other ways in the future.)