codeintel: Add panel.url and sub()

Created by: chrismwendt

• Adds panel.url to the extension context
• Adds a string substitution function sub() to the expression language

This is for the find-implementations feature to navigate to the right URL on both a Sourcegraph instance and on a code host, which is currently disabled https://github.com/sourcegraph/sourcegraph/issues/27239

Intended usage:

      {
        "id": "findImplementations",
        "title": "Find implementations",
        "command": "open",
        "commandArguments": [
          "${sub(get(context, 'panel.url'), 'panelID', 'implementations_LANGID')}"
        ]
      }

Slack