Add Nix dev environment backing the devcontainer

The devcontainer loads the project toolchain via direnv + `nix develop`,
so it needs the flake the container provisions:

- flake.nix: add k-framework, wabt, and gnumake to the devShell (the
  toolchain the project's Makefile and integration tests rely on), pin
  UV_LINK_MODE=copy.
- flake.lock: pin all flake inputs for reproducible `nix develop`.
- .envrc: `use flake` so direnv auto-loads the dev shell on entry.
- .gitignore: ignore the .direnv/ cache the container creates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: RaoulSchaffranek

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -1,3 +1,4 @@
 /dist/
 __pycache__/
 .coverage
+.direnv/