Improve Preview configuration

[jankeromnes]

• For some ports like 8088, we need to open a specific path (not just / or /index.html) like for example /vnc.html (or /ide.html for 8089). I believe we could fix this by supporting a path attribute in our port specification:

"ports": {
  "8088": {
    "label": "VNC",
    "proxy": "https",
    "preview": true,
    "path": "/vnc.html"
  }
}

(Or maybe we could somehow merge preview and path, e.g. "preview": "/vnc.html"?)

• Additionally, some Run configurations have an attribute like "openPort": "8088":

{
  "Run (dev)": {
    "cmd": "./mach run --dev",
    "openPort": "8088"
  }
}

This attribute doesn't do anything yet, but maybe we could script Cloud9 to open the right preview when this Run configuration is triggered?