Remove link to addon directory at exit of blender - take 2

[Mateusz-Grzelinski]

Design is complete. No breaking changes will be introduced in this PR.

Remove addon link at the blender exit. This is implemented as new feature with setting blender.addon.keepAddonInstalled.

In this PR implemented:

• Disable "Load Previous settings" Blender feature during VS code session. Blender breaks when trying to copy link from previous version
• Windows only: disable addon uninstallation if addon is link because if will cause data loss on Windows :
• Add option to make addon link/junction temporary blender.addon.keepAddonInstalled. If false addon will be available only during debug session
• Send notification if linking addon failed
• Update readme
• [ ] multiple blender instances might use the same addon (for example you start blender twice). Do not remove the directory when another blender instance still needs it!

Relevant: https://github.com/JacquesLucke/blender_vscode/issues/49

Pros:

• Having the link temporary will fix issues that pop up in ugly corner cases in various places:
  • uninstalling addon from blender might lead to data loss
  • importing data from previous blender converts junction (link) into directory what leads to permission error when trying to install addon

Cons:

• changes what users are used to. we can also prompt user to choose.
• removing junctions and directories in python is a mess!!! Different python versions handle junctions dramatically differently.

Design 1 - link only when debugging - used in this PR

• temporary link to addon dir is made only for VS code session - proposed in #55 but not merged, so
  • add new setting to go back to previous behaviour - simple
  • or create a command "Blender: Install addons" that links/installs addons permamently - might be complicated - addons can have different ways of installation.

Design 2 - modify addon directory - unused

• breaking change
• modify VS code startup to use separate addon BLENDER_USER_SCRIPTS and dir BLENDER_SYSTEM_EXTENSIONS
  • might conflict with user settings

Design 3

Modfiy blender preferences to recognise addon location via bpy.context.preferences.filepaths.script_directories Cons:

• breaking change
• requires specific directory structure, see instructions from this random repo: https://github.com/hextantstudios/hextant_python_debugger?tab=readme-ov-file#setup-a-custom-scripts-folder-for-your-add-on
• is extension supported via this mechanism?
• script directories change require blender restart
• older blender versions can specify one path, later have list of paths to specify in bpy.context.preferences.filepaths.script_directories - might conflict with user settings