Add draining state accessor and tests for it

src/main/php/io/streams/Buffer.class.php

@@ -37,8 +37,11 @@ public function __construct($files, int $threshold) {
   /** Returns buffer size */
   public function size(): int { return $this->size; }
 
-  /** @return ?io.File */
-  public function file() { return $this->file; }
+  /** Returns the underlying file, if any */
+  public function file(): ?File { return $this->file; }
+
+  /** Returns whether this buffer is draining */
+  public function draining(): bool { return $this->draining; }
 
   /**
    * Write a string

src/test/php/io/unittest/BufferTest.class.php

@@ -81,8 +81,10 @@ public function reset($length) {
   public function cannot_write_after_draining_started() {
     $fixture= new Buffer($this->temp, self::THRESHOLD);
     $fixture->write('Test');
+    Assert::false($fixture->draining());
 
     $fixture->read();
+    Assert::true($fixture->draining());
     Assert::throws(IllegalStateException::class, fn() => $fixture->write('Test'));
   }