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>

crates/wasmtime/src/trap.rs

@@ -42,6 +42,14 @@ impl fmt::Display for TrapReason {
 /// A trap code describing the reason for a trap.
 ///
 /// All trap instructions have an explicit trap code.
+///
+/// The code can be accessed from the c-api, where the possible values are translated
+/// into enum values defined there:
+///
+/// * `wasm_trap_code` in c-api/src/trap.rs, and
+/// * `wasmtime_trap_code_enum` in c-api/include/wasmtime/trap.h.
+///
+/// These need to be kept in sync.
 #[non_exhaustive]
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 pub enum TrapCode {