repo: add shell.nix and .envrc (gfx-rs#5519)

* repo: add shell.nix and .envrc

Makes usage on NixOS easier, by not having to drop into a hurry of
`nix-shell -p lots of packages` every time someone wants to experiment
around with wgpu.

The .envrc causes shell.nix to be eval'd automatically run iff

1. direnv is installed
2. the user `direnv allow`ed the shell.nix

* repo(shell.nix): add explanations and remove cruft

.envrc

@@ -0,0 +1 @@
+use nix

shell.nix

@@ -0,0 +1,69 @@
+# This file is only relevant for Nix and NixOS users.
+# What's actually meant by "Nix" here is not UNIX, but the *package manager* Nix, see https://nixos.org/.
+# If you are
+#   on macOS (and not using nix-darwin)
+#   or on Windows (and not using Nix in WSL),
+# you can carelessly ignore this file.
+#
+# Otherwise, if you *do* use Nix the package manager,
+# this file declares
+#   common dependencies
+#   and some nice tools
+# which you'll most likely need when working with wgpu.
+# Feel free to copy it into your own project if deemed useful.
+#
+# To use this file, just run `nix-shell` in this folder,
+# which will drop you into a shell
+# with all the deps needed for building wgpu available.
+#
+# Or if you're using direnv (https://direnv.net/),
+# use `direnv allow` to automatically always use this file
+# if you're navigating into this or a subfolder.
+
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell rec {
+  buildInputs = with pkgs; [
+    # necessary for building wgpu in 3rd party packages (in most cases)
+    libxkbcommon
+    wayland xorg.libX11 xorg.libXcursor xorg.libXrandr xorg.libXi
+    alsa-lib
+    fontconfig freetype
+    shaderc directx-shader-compiler
+    pkg-config cmake
+    mold # could use any linker, needed for rustix (but mold is fast)
+
+    libGL
+    vulkan-headers vulkan-loader
+    vulkan-tools vulkan-tools-lunarg
+    vulkan-extension-layer
+    vulkan-validation-layers # don't need them *strictly* but immensely helpful
+
+    # necessary for developing (all of) wgpu itself
+    cargo-nextest cargo-fuzz
+
+    # nice for developing wgpu itself
+    typos 
+
+    # if you don't already have rust installed through other means,
+    # this shell.nix can do that for you with this below
+    yq # for tomlq below
+    rustup
+
+    # nice tools
+    gdb rr
+    evcxr
+    valgrind
+    renderdoc
+  ];
+
+  shellHook = ''
+    export RUSTC_VERSION="$(tomlq -r .toolchain.channel rust-toolchain.toml)"
+    export PATH="$PATH:''${CARGO_HOME:-~/.cargo}/bin"
+    export PATH="$PATH:''${RUSTUP_HOME:-~/.rustup/toolchains/$RUSTC_VERSION-x86_64-unknown-linux/bin}"
+    export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${builtins.toString (pkgs.lib.makeLibraryPath buildInputs)}";
+
+    rustup default $RUSTC_VERSION
+    rustup component add rust-src rust-analyzer
+  '';
+}