c-api: add wasmtime_trap_code (#3086)

Eventually this should be added to the wasmtime-go binding, addressing
https://github.com/bytecodealliance/wasmtime-go/issues/63.

Added a snippet to examples/interrupt.c to verify that this works as
expected in manual testing.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

examples/interrupt.c

@@ -118,6 +118,10 @@ int main() {
   printf("Got a trap!...\n");
 
   // `trap` can be inspected here to see the trap message has an interrupt in it
+  wasmtime_trap_code_t code;
+  ok = wasmtime_trap_code(trap, &code);
+  assert(ok);
+  assert(code == WASMTIME_TRAP_CODE_INTERRUPT);
   wasm_trap_delete(trap);
 
   wasmtime_store_delete(store);