Add support for clickableXSD CodeLens

[fbricon]

I find that textDocument/codelens (#55) provides very little value without the ability to open reference links on the client side, that'd be computed via a call to codeLens/resolve.
The main issue so far is there's no standard way to invoke an "open references" command (that I know of). So, clients would have to implement a custom "xml.show.references" command (similar to vscode-java's own java.show.references.
Clients would need to advertise their support for that feature, in the initialization options.

[fbricon]

For reference, here's the payload returned by jdt.ls for a similar request (see the command attribute):

[Trace - 12:26:28 PM] Received response 'codeLens/resolve - (176)' in 66ms.
Result: {
    "range": {
        "start": {
            "line": 7,
            "character": 21
        },
        "end": {
            "line": 7,
            "character": 25
        }
    },
    "command": {
        "title": "5 references",
        "command": "java.show.references",
        "arguments": [
            "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
            {
                "line": 7,
                "character": 21
            },
            [
                {
                    "uri": "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
                    "range": {
                        "start": {
                            "line": 19,
                            "character": 4
                        },
                        "end": {
                            "line": 19,
                            "character": 14
                        }
                    }
                },
                {
                    "uri": "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
                    "range": {
                        "start": {
                            "line": 20,
                            "character": 4
                        },
                        "end": {
                            "line": 20,
                            "character": 14
                        }
                    }
                },
                {
                    "uri": "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
                    "range": {
                        "start": {
                            "line": 25,
                            "character": 4
                        },
                        "end": {
                            "line": 25,
                            "character": 14
                        }
                    }
                },
                {
                    "uri": "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
                    "range": {
                        "start": {
                            "line": 26,
                            "character": 4
                        },
                        "end": {
                            "line": 26,
                            "character": 14
                        }
                    }
                },
                {
                    "uri": "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
                    "range": {
                        "start": {
                            "line": 27,
                            "character": 4
                        },
                        "end": {
                            "line": 27,
                            "character": 14
                        }
                    }
                }
            ]
        ]
    },
    "data": [
        "file:///Users/fbricon/Downloads/sample-resources/foo/bar/Hello.java",
        {
            "line": 7,
            "character": 21
        },
        "references"
    ]
}

[angelozerr]

@fbricon I think we don't need in our case doing that in resolve method since command title is updated in codeLens services. Indeed the references computing for one node requires to walk inside of all nodes of DOMDocument. So the performance to find references for one node or for several nodes is the same.

In other words, we need just to update Command with the proper command in https://github.com/angelozerr/lsp4xml/blob/master/org.eclipse.lsp4xml/src/main/java/org/eclipse/lsp4xml/extensions/xsd/participants/XSDCodeLensParticipant.java#L40