fix(docs): Change the return type of format function (docs & types) (#751)

The return type was not updated when format function was changed to
an async function. This patch changes the type from `string` to
`Promise<string>`, this only affects language servers.

Author: IronBlood

src/index.js

@@ -38,7 +38,7 @@ module.exports = format;
  * @param {String} options.logLevel - the level for the logs
  *  (error, warn, info, debug, trace)
  * @param {Boolean} options.prettierLast - Run Prettier Last
- * @return {String} - the formatted string
+ * @return {Promise<String>} - the formatted string
  */
 async function format(options) {
   const { logLevel = getDefaultLogLevel() } = options;

types/index.d.ts

@@ -90,6 +90,6 @@ declare namespace format {
  * supplied configuration. The auto-formatting is limited to the issues that
  * Prettier and ESLint can automatically fix.
  */
-declare function format(options: format.Options): string;
+declare function format(options: format.Options): Promise<string>;
 
 export = format;