chore: init workspace

What does this MR do?

Creates the initial project scaffold for gitlab‑code‑parser (One Parser), delivering the minimum viable Rust workspace so downstream teams (Embeddings &  Knowledge Graph) can begin integrating the library via Cargo or FFI.

Related Issues

[Gitlab Code Parser] Create GitLab Code Parser ... (gitlab-org/gitlab#536077 - closed)

Context / Links

• Epic: One Parser (gitlab‑code‑parser)
• Related discussions & design notes:
  • API contract & chunking – #536142
  • Go bindings & packaging – #536081
  • Indexer integration PoC – #536144
  • Original proposal – #534153

Changes in this MR

Area	Change
Repo hygiene	.gitignore ignores Rust target/ artifacts
Licensing	LICENSE.md (MIT) aligns with wider Rust ecosystem & GitLab OSS policy
Workspace setup	Cargo.toml workspace + Cargo.lock (deterministic builds)
	rust-toolchain.toml pins stable toolchain with clippy/rustfmt
	mise.toml enables one‑command toolchain install (mise install)
Core crate	crates/parser-core with a trivial add() fn + unit test (CI smoke‑test)
Dependencies	Brings in ast‑grep-core, ast‑grep-config, ast‑grep-language, serde, once_cell via [workspace.dependencies] (versions locked to 0.37 / 1.0)
Docs	Seed README.md (vision, features, quick‑start) generated per epic guidance

What's next?

We can begin integrating the following into the codebase:

• Basic Parser (targets Ruby): Draft: feat: ruby parser (!1 - closed)
• Project Scaffolding and Tooling: ci: add initial tooling (gitalisk!3 - merged)

How to test locally

# Clone & build
git clone https://gitlab.com/gitlab-org/rust/gitlab-code-parser
cd gitlab-code-parser
mise install        # installs Rust toolchain
cargo test -- --nocapture      # runs parser-core unit test