src/vs/editor/common/config/editorOptions.ts/EditorFontLigatures/validate
class EditorFontLigatures:	public validate(input: any): string {
		if (typeof input === 'undefined') {
			return this.defaultValue;
		}
		if (typeof input === 'string') {
			if (input === 'false') {
				return EditorFontLigatures.OFF;
			}
			if (input === 'true') {
				return EditorFontLigatures.ON;
			}
			return input;
		}
		if (Boolean(input)) {
			return EditorFontLigatures.ON;
		}
		return EditorFontLigatures.OFF;
	}