Bundle common XML schemas

[mwisnicki]

Please bundle most common XML Schemas that are needed for features supported by this plugin such as maven-4.0.0.xsd and make them available to vscode-xml.

Currently they are downloaded from internet at runtime which is bound to fail in corporate environments because vscode-xml ignores proxy settings (redhat-developer/vscode-xml#134) and also random internet locations might not be even allowed by corporate proxies.

[rgrunber]

I think the maven xsd will be provided as part of eclipse-lemminx/lemminx#1182 , and that's probably the best place for it, so I think this can be closed as a duplicate of that issue.

As for the proxy settings, I would look at the instructions on redhat-developer/vscode-xml#764 (comment) , while also using the pre-release version of vscode-xml from the marketplace. If there's an issue still, it can be looked into there.