`DATALAB_PLUGINS` environment variable for plugin discovery

[PierreRaybaut]

Summary

Add support for a new DATALAB_PLUGINS environment variable to declare one or
more directories that should be scanned at startup, in addition to the
existing plugin search paths.

Motivation

Today, third-party plugins can only live in:

• the user plugin directory (~/.DataLab/plugins),
• the single custom directory configurable in Preferences,
• the plugins/ folder next to the executable (frozen builds),
• the internal datalab/plugins/ folder (built-ins).

This is awkward for shared/multi-user installations, CI environments,
project-scoped plugin sets and plugin authors who need to test multiple
plugin trees in parallel.

Proposed behavior

• New env var DATALAB_PLUGINS listing directories separated by os.pathsep
  (; on Windows, : on Linux/macOS), like PYTHONPATH.
• Listed directories are appended to existing plugin search paths
  (additive, not a replacement).
• Non-existent paths are silently skipped, with a warning logged.
• Read once at startup; restart required after changes.

Example:

export DATALAB_PLUGINS="/opt/lab_plugins:/home/alice/datalab_plugins"

set DATALAB_PLUGINS=C:\lab_plugins;D:\shared\datalab_plugins