Support for EditorConfig

[vitaliy-guliy]

Add an extension supporting EditorConfig to define coding styles.

Working on extension is in branch
https://github.com/theia-ide/theia/tree/editorconfig

[akosyakov]

How is it going to be integrated with the current preferences?

VS code extension overrides configurations user/workspace editor settings. Would not it make sense in our case as well?

[vitaliy-guliy]

We also override editor configuration. Editorconfig has bigger priority than editor preferences.

[akosyakov]

I meant we should consider whether we want to make sure that the preferences API returns consistent results with what users see in editors. If so then we should hook it up in the preferences machinery.

cc @svenefftinge