Provide encoding-related APIs for editor extensions

[bpasero]

for #824

export interface TextDocument {

	readonly encoding: string;

	save(options?: { encoding?: string }): Thenable<boolean>;
}

export namespace workspace {
	export function openTextDocument(uri: Uri, options?: { encoding?: string }): Thenable<TextDocument>;
	export function openTextDocument(path: string, options?: { encoding?: string }): Thenable<TextDocument>;
}

[lygstate]

long waiting, thanks!