Add an ArgumentParser helper for integers

Summary:
Ref T13676. With increased PHP8.1 strictness around null, it seems reasonable to add some convenience parsing to "PhutilArgumentParser".

Add a helper function for parsing integers.

Test Plan:
See next change. Tried these arguments:

```
--count ''
--count asdf
--count 0
--count -3
--count 999999999999999999999999999999999999999999999234
--count 5
```

Got sensible parsing and appropriate feedback in all cases.

Maniphest Tasks: T13676

Differential Revision: https://secure.phabricator.com/D21799

src/parser/argument/PhutilArgumentParser.php

@@ -625,6 +625,33 @@ final class PhutilArgumentParser extends Phobject {
     return $this->specs[$name]->getDefault();
   }
 
+  public function getArgAsInteger($name) {
+    $value = $this->getArg($name);
+
+    if ($value === null) {
+      return $value;
+    }
+
+    if (!preg_match('/^-?\d+\z/', $value)) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Parameter provided to argument "--%s" must be an integer.',
+          $name));
+    }
+
+    $intvalue = (int)$value;
+
+    if (phutil_string_cast($intvalue) !== phutil_string_cast($value)) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Parameter provided to argument "--%s" is too large to '.
+          'parse as an integer.',
+          $name));
+    }
+
+    return $intvalue;
+  }
+
   public function getUnconsumedArgumentVector() {
     return $this->argv;
   }