Streamline configuration schema

[jason0x43]

The structure and patterns of Intern's configuration schema should be made more consistent. In particular, related items should be grouped, and some items could be better named.

Group all functional* items under a new webdriver section:

{
  "webdriver": {
    "suites": ["tests/webdriver/*.ts"],
    "baseUrl": ".",
    "coverage": false,
    "retries": 3,
    "timeouts": {
      ...
    }
}

Combine tunnel and tunnelOptions:

{
  "tunnel": {
    "name": "selenium",
    "options": { ... }
  }
}

Combine benchmark and benchmarkConfig:

{
  "benchmark": {
    "enabled": false,
    "options": { ... }
  }
}

See #1041 for discussion.