add xml.fileAssociations configuration item

[yosuke]

This patch will add xml.fileAssociations configuration item to the extension.

xml.fileAssociations will enable us to associate files with specific extension to the XSD. In my use case, I use this to edit ROS configuration files:

   "files.associations": {
        "*.launch": "xml",
        "*.urdf.xacro": "xml",
        "*.urdf": "xml",
        "model.config": "xml",
        "*.sdf": "xml"
    },
    "xml.fileAssociations": [
        {
            "systemId": "http://download.ros.org/schema/package_format2.xsd",
            "pattern": "package.xml"
        },
        {
            "systemId": "https://gist.githubusercontent.com/nalt/dfa2abc9d2e3ae4feb82ca5608090387/raw/roslaunch.xsd",
            "pattern": "**/*.launch"
        },
        {
            "systemId": "https://raw.githubusercontent.com/ros/urdfdom/master/xsd/urdf.xsd",
            "pattern": "**/*.urdf.xacro"
        },
        {
            "systemId": "https://raw.githubusercontent.com/ros/urdfdom/master/xsd/urdf.xsd",
            "pattern": "**/*.urdf"
        },
        {
            "systemId": "http://sdformat.org/schemas/root.xsd",
            "pattern": "**/*.sdf"
        }
    ]