Bump syn from 1.0.83 to 1.0.84

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.83 to 1.0.84.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@1.0.83...1.0.84)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: indirect
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>