Provide validation based on an xsd

[apupier]

No description provided.

[angelozerr]

Indeed. https://github.com/microclimate-devops/xml-language-server/tree/master/server/xml-server seems doing that.

[angelozerr]

It starts working, but a lot of features to implement again like

• manage better start/end location for error (because XSD validation from xerces manages just a start position and not a range)
• cache XML Schema, because for the moment, the XML Schema is loaded each time you type something. But there is not freeze because publish diagnostics is done in async mode. To fix that, I think XML catalog should help us and I need to read https://xerces.apache.org/xerces2-j/faq-grammars.html

[angelozerr]

I close this issue, since validation is working by using Xerces. We need just adjust ranges in #71 and #72

For cache topic, see #73