fix: types for loadConfig and add more tests (#2123)

Author: SethFalco

lib/svgo-node.js

@@ -37,7 +37,7 @@ export * from './svgo.js';
  * You can also specify relative or absolute path and customize current working
  * directory.
  *
- * @type {<T extends string>(configFile?: T | null, cwd?: string) => Promise<T extends string ? import('./svgo.js').Config : import('./svgo.js').Config | null>}
+ * @type {<T extends string | null>(configFile?: T, cwd?: string) => Promise<T extends string ? import('./svgo.js').Config : import('./svgo.js').Config | null>}
  */
 export const loadConfig = async (configFile, cwd = process.cwd()) => {
   if (configFile != null) {

test-d/lib/svgo-node.test-d.ts

@@ -1,5 +1,16 @@
 import { expectType, expectAssignable } from 'tsd';
-import { DataUri, Output, optimize } from '../../types/lib/svgo-node.js';
+import {
+  type Config,
+  type DataUri,
+  type Output,
+  loadConfig,
+  optimize,
+} from '../../types/lib/svgo-node.js';
 
 expectType<Output>(optimize('<svg></svg>'));
 expectAssignable<DataUri>('enc');
+
+expectType<Promise<Config | null>>(loadConfig());
+expectType<Promise<Config | null>>(loadConfig(undefined));
+expectType<Promise<Config | null>>(loadConfig(null));
+expectType<Promise<Config>>(loadConfig('svgo.config.js'));