add support for XML catalogs (schema matching: matching, rewriting, ...)

[GitMensch]

The catalog entries (replace one entry by another) via xmlComplete.schemaMapping works fine and is quite simiar to eclipse's catalog entries:

 {
  "xmlns": "https://github.com/avaloniaui",
  "xsdUri": "https://raw.githubusercontent.com/rogalmic/vscode-xml-complete/master/test/Avalonia/AvaloniaXamlSchema.xsd"
 }

It would be very ood if we could have rewrite options, which is especially usefiul for local files. This feature is available with the standardized xml catalog. The eclipse IDE seems to just wrap this one in a nice editor.

For the setting, we can just add a new one or even better use the xml.catalogs array, see
https://github.com/redhat-developer/vscode-xml/wiki/XMLValidation#xml-catalog-with-xsd.