Rust: copy files from rust-analyzer's codegenerator

Files copied from: https://github.com/rust-lang/rust-analyzer/tree/50882fbfa204027c84753e6d51a1a12884dc1b19/xtask/src/codegen/grammar

Author: aibaars

rust/generate-schema/Cargo.toml

@@ -5,4 +5,7 @@ edition = "2021"
 
 [dependencies]
 ungrammar = "1.16.1"
+proc-macro2 = "1.0.33"
+quote = "1.0.12"
+itertools = "0.10.1"