Update syn requirement from 1.0.109 to 2.0.15

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@1.0.109...2.0.15)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
...

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

Author: dependabot[bot]

macros/Cargo.toml

@@ -14,4 +14,4 @@ proc-macro = true
 mlua = { version = "0.8.7", features = ["luajit", "vendored"] }
 proc-macro2 = "1.0.51"
 quote = "1.0.23"
-syn = { version = "1.0.109", features = ["full"] }
+syn = { version = "2.0.15", features = ["full"] }