Label provider should take into account a language matching to the given URI

[tolusha]

Description

Label provider [1] should take into account a language matching to the given URI [2] to create an icon, not only a file extension like right now (i.e. UI should reflect mime associations, e.g. explorer should show proper a matching language icon)

[1] https://github.com/theia-ide/theia/blob/master/packages/core/src/browser/label-provider.ts#L27https://github.com/theia-ide/theia/blob/master/packages/filesystem/src/browser/filesystem-frontend-contribution.ts#L161
[2] https://github.com/theia-ide/theia/blob/master/packages/core/src/browser/mime-service.ts#L25

Reference issue

#4581