Run with zero config

[jason0x43]

Intern should be runnable with no configuration. When no config is provided, the suites list should use a default value, and the node environment should be used.

Currently, Intern only requires a minimal config to run:

{
  "suites": "tests/**/*.js"
}

We can eliminate even that need by defaulting the suites list to a value that would handle many common situations, like:

  ["tests/**/*.ts", "src/**/*.spec.ts"]

The extension could be chosen based on the presence of a tsconfig file in the project root / cwd.

If an intern.json file is added to a project, any zero-config behaviors should be disabled.