Improve exporting

[isc-bsaviano]

This PR fixes #129

Changes made:

• Modify "vscode-objectscript.export" to do the following:
  • Don’t allow the user to select a virtual folder as the export destination.
  • If there are no suitable folders in the workspace, notify the user and exit.
  • Replace the call to AtelierAPI.getDocNames() with a call to the StudioOpenDialog query to improve performance and to allow the filtering out of system items when the namespace is not %SYS.
  • Ask the user for confirmation and allow them to exclude individual files from the export list before starting the export.
• Add "vscode-objectscript.exportCurrentFile” command that exports the currently opened file, if it was opened from the ObjectScript Explorer.
• Change RootNode.getList() query to only SELECT needed columns from StudioOpenDialog to improve performance.
• Improve documentation for "objectscript.export.filter" setting.

[gjsjohnmurray]

Various comments for consideration.