Improve file / folder selection for some vscode-xml settings

vscode-xml provides several settings with file (ex : xml.catalogs, xml.java.home) or folder (ex : xml.server.workDir). It should be fine to provide command to select a file or a folder that we could call in the settings with an hyperlink (like it is done for documentation in https://github.com/redhat-developer/vscode-xml/pull/327 with command: syntax.

It will fix for instance user requirement:

In the extension settings page, when clicking on the Add button to add a new catalog, there should be a Browse button to allow the user to browse the file system and select the desired catalog rather than having to paste a string in the text box which is very error prone
See https://github.com/redhat-developer/vscode-xml/issues/315#issue-687523227

Start by examining the extension's VS Code settings and command contributions, focusing on the file settings such as xml.catalogs and xml.java.home and the folder setting xml.server.workDir. Compare the proposed command-link approach with the documentation link syntax referenced in pull request 327 and the catalog workflow described in issue 315. Done means users can invoke file or folder selection from the relevant settings.