Adjust how async readFile is loaded

`fs/promises` isn't available in node 12

Author: xt0rted

src/dotnet.ts

@@ -1,4 +1,4 @@
-import { readFile } from "fs/promises";
+import { promises } from "fs";
 import { resolve } from "path";
 
 import {
@@ -14,6 +14,8 @@ import { getPullRequestFiles } from "./files";
 
 import type { DotNetFormatVersion } from "./version";
 
+const { readFile } = promises;
+
 export type FormatFunction = (options: FormatOptions) => Promise<boolean>;
 
 export interface FormatOptions {