[plug-in]Support VS Code XML Tooling

[apupier]

currently it is failing with:

root ERROR [hosted-plugin: 211] (node:211) UnhandledPromiseRejectionWarning: SyntaxError: Unexpected token u in JSON at position 0
    at JSON.parse (<anonymous>)
    at getSettings (/tmp/vscode-unpacked/vscode_extension_redhat_vscode_xml.vsix/extension/out/src/extension.js:74:29)
    at requirements.resolveRequirements.catch.then.requirements (/tmp/vscode-unpacked/vscode_extension_redhat_vscode_xml.vsix/extension/out/src/extension.js:45:48)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

the corresponding code is:

function getSettings() {
        let configXML = vscode_1.workspace.getConfiguration();
        configXML = configXML.get('xml');
        let x = JSON.stringify(configXML);
        let settings = JSON.parse(x);
        settings['logs']['file'] = logfile;
        return settings;
    }

Line 74 is the JSON.parse

[apupier]

//cc @benoitf

[apupier]

seems that it is now another error: eclipse-che/che#12536

[vrubezhny]

seems that it is now another error: eclipse/che#12536

This error is going to be addressed in #3540, I've tested the fix referenced from #3540 549668d and it looks like it does the thing: I was able to deploy vscode:extension/redhat.vscode-xml with no TypeError: Cannot read property 'editor.autoClosingBrackets' of undefined error appeared. However there is no PR made yet for #3540 - it is still in progress.

UPD: There is a PR created for #3540: #4296

[vrubezhny]

After the PR for #3540 is merged, VSCode XML extension plug-in can be successfully deployed into theia and works fine.
I believe we can close this issue.