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>

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1759831965,
+        "narHash": "sha256-vgPm2xjOmKdZ0xKA6yLXPJpjOtQPHfaZDRtH+47XEBo=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c9b6fb798541223bbb396d287d16f43520250518",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}