Make decoration provider API public

[sandy081]

Make the following API public

export interface DecorationProvider {
	onDidChangeDecorations: Event<undefined | Uri | Uri[]>;
	provideDecoration(uri: Uri, token: CancellationToken): ProviderResult<DecorationData>;
}

export namespace window {
	export function registerDecorationProvider(provider: DecorationProvider): Disposable;
}

[eamodio]

Instead of DecorationData, what about just Decoration or UriDecoration or ViewDecoration or TreeViewDecoration or something like that?

[jrieken]

Yeah, +1 for Decoration. No matter what we chose it should be aligned with the provide-function name.