Re-export v8 use_custom_libcxx feature

Editor assessment

Two blemishes on an otherwise ordinary header. First, 'Re-export' starts with a capital letter where the spec's examples and broad convention keep subjects lowercase; trivial, but humans and linters notice. Second, the word 'feature' is ambiguous — here it means a Cargo feature (v8's use_custom_libcxx), yet in a conventional-commits context 'feature' reads as user-facing functionality, making feat seem plausible when this is really a build/configuration concern; build would arguably fit better. Backticks around `use_custom_libcxx` would also disambiguate. The scope is fine and the length is short. A clearer frame: 'build(core): re-export v8 use_custom_libcxx cargo feature'.