Add proc macro crate dependencies (syn, proc-macro2) (#550)

## What is the goal of this PR?

We add dependencies on `syn` and `proc-macro2`, the ubiquitous
procedural macro crates.

Currently they are only used for grammar tests in TypeQL3, but they will
almost certainly be used for our own proc-macros and so are placed under
`dependencies` rather than `dev-dependencies`.

Author: dmitrii-ubskii