fix: Node polyfill fsAppend rework (#4322)

* My original implementation of `fs.appendFile` used an async API, which, though 
  it would work fine as a polyfill, wasn't an exact match with the Node API.  This PR
  reworks that API to mimic the Node API fully as a synchronous void function with
  an async internal implementation.
* Refactor move of other internal fs `dirent` and `dir` classes to the _fs internal
  directory.

std/node/_fs/_fs_dir_test.ts

@@ -0,0 +1,156 @@
+const { test } = Deno;
+import { assert, assertEquals, fail } from "../../testing/asserts.ts";
+import Dir from "./_fs_dir.ts";
+import Dirent from "./_fs_dirent.ts";
+
+test({
+  name: "Closing current directory with callback is successful",
+  async fn() {
+    let calledBack = false;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    new Dir(".").close((valOrErr: any) => {
+      assert(!valOrErr);
+      calledBack = true;
+    });
+    assert(calledBack);
+  }
+});
+
+test({
+  name: "Closing current directory without callback returns void Promise",
+  async fn() {
+    await new Dir(".").close();
+  }
+});
+
+test({
+  name: "Closing current directory synchronously works",
+  async fn() {
+    new Dir(".").closeSync();
+  }
+});
+
+test({
+  name: "Path is correctly returned",
+  fn() {
+    assertEquals(new Dir("std/node").path, "std/node");
+
+    const enc: Uint8Array = new TextEncoder().encode("std/node");
+    assertEquals(new Dir(enc).path, "std/node");
+  }
+});
+
+test({
+  name: "read returns null for empty directory",
+  async fn() {
+    const testDir: string = Deno.makeTempDirSync();
+    try {
+      const file: Dirent | null = await new Dir(testDir).read();
+      assert(file === null);
+
+      let calledBack = false;
+      const fileFromCallback: Dirent | null = await new Dir(
+        testDir
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      ).read((err: any, res: Dirent) => {
+        assert(res === null);
+        assert(err === null);
+        calledBack = true;
+      });
+      assert(fileFromCallback === null);
+      assert(calledBack);
+
+      assertEquals(new Dir(testDir).readSync(), null);
+    } finally {
+      Deno.removeSync(testDir);
+    }
+  }
+});
+
+test({
+  name: "Async read returns one file at a time",
+  async fn() {
+    const testDir: string = Deno.makeTempDirSync();
+    Deno.createSync(testDir + "/foo.txt");
+    Deno.createSync(testDir + "/bar.txt");
+
+    try {
+      let secondCallback = false;
+      const dir: Dir = new Dir(testDir);
+      const firstRead: Dirent | null = await dir.read();
+      const secondRead: Dirent | null = await dir.read(
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        (err: any, secondResult: Dirent) => {
+          assert(
+            secondResult.name === "bar.txt" || secondResult.name === "foo.txt"
+          );
+          secondCallback = true;
+        }
+      );
+      const thirdRead: Dirent | null = await dir.read();
+
+      if (firstRead?.name === "foo.txt") {
+        assertEquals(secondRead?.name, "bar.txt");
+      } else if (firstRead?.name === "bar.txt") {
+        assertEquals(secondRead?.name, "foo.txt");
+      } else {
+        fail("File not found during read");
+      }
+      assert(secondCallback);
+      assert(thirdRead === null);
+    } finally {
+      Deno.removeSync(testDir, { recursive: true });
+    }
+  }
+});
+
+test({
+  name: "Sync read returns one file at a time",
+  fn() {
+    const testDir: string = Deno.makeTempDirSync();
+    Deno.createSync(testDir + "/foo.txt");
+    Deno.createSync(testDir + "/bar.txt");
+
+    try {
+      const dir: Dir = new Dir(testDir);
+      const firstRead: Dirent | null = dir.readSync();
+      const secondRead: Dirent | null = dir.readSync();
+      const thirdRead: Dirent | null = dir.readSync();
+
+      if (firstRead?.name === "foo.txt") {
+        assertEquals(secondRead?.name, "bar.txt");
+      } else if (firstRead?.name === "bar.txt") {
+        assertEquals(secondRead?.name, "foo.txt");
+      } else {
+        fail("File not found during read");
+      }
+      assert(thirdRead === null);
+    } finally {
+      Deno.removeSync(testDir, { recursive: true });
+    }
+  }
+});
+
+test({
+  name: "Async iteration over existing directory",
+  async fn() {
+    const testDir: string = Deno.makeTempDirSync();
+    Deno.createSync(testDir + "/foo.txt");
+    Deno.createSync(testDir + "/bar.txt");
+
+    try {
+      const dir: Dir = new Dir(testDir);
+      const results: Array<string | null> = [];
+
+      for await (const file of dir[Symbol.asyncIterator]()) {
+        results.push(file.name);
+      }
+
+      assert(results.length === 2);
+      assert(results.includes("foo.txt"));
+      assert(results.includes("bar.txt"));
+    } finally {
+      Deno.removeSync(testDir, { recursive: true });
+    }
+  }
+});