fix: lintFiles() to await on ESLint.outputFixes() (#318)

This ensures that fixes are written before resolving.

Without this, the file may be in a weird state if consumed right after calling lintFiles() (e.g. reading the files right after calling lintFiles() would return an empty string, stat would show a zero-sized file).

index.js

@@ -89,7 +89,7 @@ class StandardEngine {
     const result = await eslintInstance.lintFiles(files)
 
     if (eslintConfig.fix) {
-      this.eslint.ESLint.outputFixes(result)
+      await this.eslint.ESLint.outputFixes(result)
     }
 
     return result