Add onunhandledrejection error machanism type to rejected promises

[romtsn]

Description

Currently we override whatever mechanism comes from the JS sdk, see here:

sentry-react-native/src/js/integrations/reactnativeerrorhandlers.ts

Lines 146 to 160 in c2a4e9b

	if (isFatal) {
	event.level = 'fatal' as SeverityLevel;
	addExceptionMechanism(event, {
	handled: false,
	type: 'onerror',
	});
	} else {
	event.level = 'error';
	addExceptionMechanism(event, {
	handled: true,
	type: 'generic',
	});
	}

Perhaps we should preserve the original mechanism type at least, because it might be useful when filtering issues in issue stream/discover. One such case was reported with regards to onunhandledrejection type for unhandled promise rejections.

[krystofwoldrich]

We create a new event in the global error handler and we set the error mechanism, there is no previous value.

onunhandledrejection is handled at:

sentry-react-native/packages/core/src/js/integrations/reactnativeerrorhandlers.ts

Lines 81 to 85 in 933804e

	captureException(error, {
	data: { id },
	originalException: error,
	syntheticException: isErrorLike(error) ? undefined : createSyntheticError(),
	});

And I agree we should add the correct mechanism to it.