Add a specific `xml.java.home` preference

[fbricon]

currently, the XML language server falls back to the "java.home" preference to launch.
This causes 2 issues:

• when the Java extension is not installed, the "java.home" preference is unknown, see Unknown Configuration setting #135
• you can't use a JRE to launch the server, as the Java extension doesn't support it, see JRE support #136

Introducing new "xml.java.home" preference which would be checked before "java.home", would solve #135 and help finalizing #136