refactor(playground): Make SENTRY_MIDDLEWARE_PATH non-exported

Only the derived constants are used externally.

Author: antonis

packages/core/src/js/metro/constants.ts

@@ -1,3 +1,3 @@
-export const SENTRY_MIDDLEWARE_PATH = '__sentry';
+const SENTRY_MIDDLEWARE_PATH = '__sentry';
 export const SENTRY_CONTEXT_REQUEST_PATH = `${SENTRY_MIDDLEWARE_PATH}/context`;
 export const SENTRY_OPEN_URL_REQUEST_PATH = `${SENTRY_MIDDLEWARE_PATH}/open-url`;