Add Nix flake for development environment (#23406)

Provides a Nix flake as an alternative to `scripts/bootstrap.sh` for
setting up the Bun development environment.

## What's included:

- **flake.nix**: Full development environment with all dependencies from
bootstrap.sh
  - LLVM 19, CMake 3.30+, Node.js 24, Rust, Go
  - Build tools: ninja, ccache, pkg-config, make
  - Chromium dependencies for Puppeteer testing
  - gdb for core dump debugging

- **shell.nix**: Simple wrapper for `nix-shell` usage

- **cmake/CompilerFlags.cmake**: Nix compatibility fixes
  - Disable zstd debug compression (Nix's LLVM not built with zstd)
  - Set _FORTIFY_SOURCE=0 for -O0 debug builds
  - Downgrade _FORTIFY_SOURCE warning to not error

## Usage:

```bash
nix-shell
export CMAKE_SYSTEM_PROCESSOR=$(uname -m)
bun bd
```

## Verified working:
✅ Successfully compiles Bun debug build
✅ Binary tested: `./build/debug/bun-debug --version` → 1.2.24-debug
✅ All dependencies from bootstrap.sh included

## Advantages:
- Fully isolated (no sudo required)
- 100% reproducible dependency versions  
- Fast setup with binary caching

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

CONTRIBUTING.md

@@ -2,7 +2,21 @@ Configuring a development environment for Bun can take 10-30 minutes depending o
 
 If you are using Windows, please refer to [this guide](https://bun.com/docs/project/building-windows)
 
-## Install Dependencies
+## Using Nix (Alternative)
+
+A Nix flake is provided as an alternative to manual dependency installation:
+
+```bash
+nix develop
+# or explicitly use the pure shell
+# nix develop .#pure
+export CMAKE_SYSTEM_PROCESSOR=$(uname -m)
+bun bd
+```
+
+This provides all dependencies in an isolated, reproducible environment without requiring sudo.
+
+## Install Dependencies (Manual)
 
 Using your system's package manager, install Bun's dependencies: