feat: add wrenGetValueDouble API and benchmark for wrenCall() performance

Add a new public API function wrenGetValueDouble() to extract numeric values from WrenValue handles, with proper assertions for NULL and type checking. Introduce a comprehensive benchmark test for wrenCall() that measures invocation overhead by calling a foreign method 1,000,000 times across a separate VM instance, validating result correctness and reporting elapsed time. Include the benchmark script api_call.wren and register it in the Python benchmarking harness. Also fix minor assertion message typos in wren_vm.c for consistency.

test/benchmark/api_call.wren

@@ -0,0 +1,9 @@
+class Benchmark {
+  foreign static call(iterations)
+}
+
+var result = Benchmark.call(1000000)
+// Returns false if it didn't calculate the right value. Otherwise returns the
+// elapsed time.
+System.print(result is Num)
+System.print("elapsed: %(result)")