Add remote mode for initial Remote Development compatibility (#1861)

## Changes

https://docs.databricks.com/aws/en/dev-tools/ssh-tunnel

When running inside a Databricks Remote SSH session
(DATABRICKS_REMOTE_ENV=1 and DATABRICKS_VIRTUAL_ENV set), the extension
now activates in a minimal mode: it sets the Python virtual env path,
registers a placeholder tree view, and disables all Databricks-specific
commands/views/menus via the new `databricks.context.remoteMode`
when-context.


## Tests

Manually + plus new unit tests

Author: ilia-db