[plug-in] Implement 'configurationDefault' contribution point

[akurinnoy]

contributes.configurationDefaults

Configuration example:

"contributes": {
    "configurationDefaults": {
        "[markdown]": {
            "editor.wordWrap": "on",
            "editor.quickSuggestions": false
        }
    }
}

[akosyakov]

@akurinnoy Are you working on it?

[akurinnoy]

@akosyakov not yet

[akosyakov]

@akurinnoy Is there a time frame when it can be implemented? It is needed for some built-in extensions and to fix vscode api tests.

[akurinnoy]

@akosyakov unfortunately, we cannot take it in following sprint, but, we could do it in next sprint in 3 weeks, I think

[akosyakov]

@akurinnoy Would it be alright if i look at it now?

[akurinnoy]

@akosyakov Sure! Should I re-assign the issue?