fix #443 support more ${...} variables in server menu links

[gjsjohnmurray]

This PR fixes #443

New variables are:

• ${serverUrl} - for example http://localhost:52773
• ${serverAuth} - for example &IRISUsername=_SYSTEM&IRISPassword=SYS
• ${ns} - the raw ns parameter of the connection. Different from the already-defined ${namespace} variable.

An example of how the new capabilities can launch WebTerminal:

    "objectscript.conn": {
        "server": "vscodelab",
        "ns": "%SYS",
        "active": true,
        "links": {
            "$(terminal) WebTerminal": "${serverUrl}/terminal/?ns=${ns}${serverAuth}"
        }
    }