[Gitlab Code Parser] Create GitLab Code Parser Project

Problem to solve

The One Parser epic needs a compilable Rust workspace so feature teams can begin integrating and benchmarking the library. Without a scaffold, every consumer is blocked.

 Proposal

1. Create a top‑level Cargo workspace:
   • Cargo.toml with members = ["crates/parser-core"]
   • Lock shared versions of ast‑grep-*, serde, once_cell.
2. Add minimal parser-core crate:
   • Expose placeholder API (pub fn add(a,b) -> i32) with unit test to exercise CI.
3. Repository hygiene & tooling:
   • .gitignore for target/
   • LICENSE.md (MIT)
   • rust-toolchain.toml pinning stable + rustfmt, clippy
   • mise.toml for one‑shot tool install.
4. Author a concise README.md covering vision, features, setup.