Add fluent-syntax dependency

This dependency is needed for AST traversal and validation of Fluent
resources. It allows parsing and analyzing the structure of FTL files
to detect validation errors at compile time.

Author: kevin-hua-kraken

Cargo.lock

@@ -13,5 +13,6 @@ pyo3 = { version = "0.25.1", features = ["chrono"] }
 fluent = "0.17.0"
 unic-langid = "0.9.6"
 fluent-bundle = "0.16.0"
+fluent-syntax = "0.12.0"
 chrono = "0.4.41"
 miette = { version = "7.6.0", features = ["fancy"] }