vscode: Support opening local documentation if available

Displaying local instead of web docs can have many benefits:
- the web version may have different features enabled than locally selected
- the standard library may be a different version than is available online
- the user may not be online and therefore cannot access the web documentation
- the documentation may not be available online at all, for example because it
  is for a new feature in a library the user is currently developing

If the documentation is not available locally, the extension still falls back to
the web version.

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