fix: support separator char in envvar default value

[acolombier]

This fix allows support for default environment variable containing a :, used as a sepeartor. A common example for this use case is

  ...
  "remoteEnv": {
    "DISPLAY": "${localEnv:DISPLAY::0}",
  }
  ...

Without this fix, DISPLAY will be set to an empty string (basically won't be set) if no DISPLAY variable is defined on the host. With this fix it will default to :0