LibRegex: Reduce RegexMatcher's BumpAllocator chunk size

Before the BumpAllocator OOB access issue was understood and fixed, the
chunk size was increased to 8MiB as a workaround in commit:
27d555bab0.

The issue is now resolved by: 0f1425c895.

We can reduce the chunk size to 2MiB, which has the added benefit of
reducing runtime of the RegExp.prototype.exec test.

Userland/Libraries/LibRegex/RegexMatcher.cpp

@@ -391,7 +391,7 @@ private:
         Node* previous { nullptr };
     };
 
-    UniformBumpAllocator<Node, true, 8 * MiB> m_allocator;
+    UniformBumpAllocator<Node, true, 2 * MiB> m_allocator;
     Node* m_first { nullptr };
     Node* m_last { nullptr };
 };