FR: for new commands "validate all XML in workspace/folder"

Scenario 1: the application in the workspace creates a bunch of XML files.
--> it would be nice to be able to validate all "on the fly" and have the problem pane updated with the results

Scenario 2: a new version of the xsd is distributed, all files have to be validated to check for necessary adjustments.

Current workaround:

• use the file explorer (or [CTRL]+[O]) to open all files within vscode
• issue xml.validation.all.files (validate all opened XML files)
• close all the files that are fine by hand, which can take a while

Suggestion:

• xml.validation.workspace.files -> would validate all files found in the workspace
• xml.validation.folder.files -> would ask for a folder and validate all files found in there (could be in the workspace or out of it)

Start by tracing the existing xml.validation.all.files command and its validation results. Define how workspace-wide and folder-selected XML files are discovered and validated, then verify that diagnostics update the problem pane and that both proposed commands handle files outside the workspace when a folder is chosen.