Improve tsconfig.json

[harmony7]

This PR enhances the project's TypeScript settings:

• Uses moduleResolution: bundler to enable conditional imports and exports
  This is described in this article and works better for our use case because it enables:
  • Named imports and named exports
  • Custom conditions (see below)

• Add "fastly" custom condition to match js-compute-runtime
  • js-compute-runtime adds this condition in its "bundle" step so it makes sense to allow TypeScript to resolve these as well.
• Remove skipLibCheck as that is no longer needed
  • This was only needed when Fastly Compute's types file conflicted with Node.js in the past.