src/vs/editor/common/services/modelService.ts/ModelService/setMode
class ModelService:	public setMode(model: ITextModel, languageSelection: ILanguageSelection): void {
		if (!languageSelection) {
			return;
		}
		const modelData = this._models[MODEL_ID(model.uri)];
		if (!modelData) {
			return;
		}
		modelData.setLanguage(languageSelection);
	}