Add detection for Hack files with ".hh" file extension

Hack is Facebook's dialect of PHP: http://hacklang.org/. This adds support for detecting it via the ".hh" file extension; although that extension techincally conflicts with C++ headers, the files look different enough that the existing classifier based on sample code has no trouble distinguising them.

This diff deliberately does not deal with detecting ".php" as another valid extension for Hack code. That's much trickier since the code looks basically identical to PHP to the classifier, and needs a different approach.

samples/Hack/FakeDB.hh

@@ -0,0 +1,65 @@
+<?hh // strict
+
+type DBResultExtra = shape('age' => int);
+type DBResult = shape(
+  'id' => int,
+  'name' => string,
+  'extra' => DBResultExtra,
+);
+
+final class FakeDB {
+  public function getRawRows(): array<array<string, mixed>> {
+    $good_extra = json_encode(array('age' => 40));
+    $bad_extra = 'corrupt data';
+    // Real code would query a DB, but for now let's hardcode it
+    return array(
+      array(
+        'id' => 123,
+        'name' => 'Alice',
+        'extra' => $good_extra,
+      ),
+      array(
+        'id' => 456,
+        'name' => 'Bob',
+        'extra' => $bad_extra,
+      ),
+    );
+  }
+
+  /**
+   * When processing untyped data you need to check each piece of data and
+   * figure out whether to give up or recover when the data is bad
+   */
+  public function processRow(array<string, mixed> $row): ?DBResult {
+    $row = Map::fromArray($row);
+    $id = $row->contains('id') ? $row['id'] : null;
+    $name = $row->contains('name') ? $row['name'] : null;
+    $extra = $row->contains('extra') ? json_decode($row['extra'], true) : null;
+
+    // Ignore rows with invalid IDs or names
+    if (!is_int($id) || !is_string($name)) {
+      return null;
+    }
+
+    // Try to recover from a bad extra column
+    if (!is_array($extra)) {
+      $extra = shape('age' => 0);
+    } else {
+      $extra = Map::fromArray($extra);
+      $extra = shape('age' => $extra->contains('age') ? $extra['age'] : 0);
+    }
+
+    return shape('id' => $id, 'name' => $name, 'extra' => $extra);
+  }
+
+  public function getDBResults(): Vector<DBResult> {
+    $ret = Vector {};
+    foreach ($this->getRawRows() as $raw_row) {
+      $row = $this->processRow($raw_row);
+      if ($row !== null) {
+        $ret->add($row);
+      }
+    }
+    return $ret;
+  }
+}