Add vendored dependencies & cleanup script

mach/README.md

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+add-dependencies.sh & README.md originally from https://github.com/hexops/dawn
+
+## Updating
+
+Process for updating Dawn:
+
+1. `git remote add upstream https://dawn.googlesource.com/dawn`
+2. `git checkout main && git checkout -B update-nov-30` (replace date with current date)
+3. `git fetch upstream && git merge upstream/main`
+4. `rm -rf out/ third_party/ build/ && git checkout upstream/main -- third_party/`
+5. `gclient sync` At this point there will be quite a large diff as many `third_party/` dependencies we commit will have changed.
+6.  Stage and review the changes to the `third_party/` directory by running `./mach/add-dependencies.sh`. Alter the script to include/exclude files as needed, commit the result. Make sure test files, documentation, `.gitmodules`, etc. are not included.

mach/add-dependencies.sh

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+#!/usr/bin/env bash
+set -exuo pipefail
+
+echo "Note: You should run gclient sync before this script"
+
+# Remove unneeded dependencies
+rm -rf build
+# rm -rf build_overrides
+rm -rf buildtools
+# rm -rf infra
+rm -rf testing
+rm -rf third_party/angle
+rm -rf third_party/catapult
+rm -rf third_party/googletest
+rm -rf third_party/llvm-build
+rm -rf third_party/markupsafe
+rm -rf third_party/swiftshader
+rm -rf third_party/vulkan_memory_allocator
+rm -rf third_party/vulkan-deps/glslang
+rm -rf third_party/vulkan-deps/spirv-cross
+rm -rf third_party/vulkan-deps/vulkan-headers
+rm -rf third_party/vulkan-deps/vulkan-loader
+rm -rf third_party/vulkan-deps/vulkan-tools
+rm -rf third_party/vulkan-deps/vulkan-validation-layers
+rm -rf third_party/zlib
+
+# Remove gitmodules, some third_party/ repositories contain these and leaving them around would
+# cause any recursive submodule clones to fail because e.g. some reference internal Google
+# repositories. We don't want them anyway.
+find third_party -type f -name .gitmodules -delete
+
+# Turn subrepositories into regular folders.
+find third_party -depth -type d -name .git -exec rm -rf {} \;
+
+# Remove files that are not needed.
+find third_party -depth -type d -name tests -print -exec rm -rf {} \;
+find third_party -depth -type d -name docs -print -exec rm -rf {} \;
+find third_party -depth -type d -name samples -print -exec rm -rf {} \;
+rm -rf third_party/tint/test/
+rm -rf third_party/swiftshader/third_party/SPIRV-Tools # already in third_party/vulkan-deps/spirv-tools
+rm -rf third_party/swiftshader/third_party/SPIRV-Headers # already in third_party/vulkan-deps/spirv-headers
+
+# Remove gn/gni files
+# find . -type f -name '*.gn' -delete
+# find . -type f -name '*.gni' -delete
+
+git add third_party/
+echo "you may now 'git commit -s -m 'update dependencies' if you are happy with the staged changes"