Add copilot environment configuration.

.github/copilot-instructions.md

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+## Configure CMake and Build
+
+To configure cmake, run `cmake --preset default --fresh`.
+To build, run `cmake --workflow --preset debug` or `cmake --workflow --preset release`.
+
+## Formatting
+
+Your PR needs to be formatted according to our coding style.
+Run `./extras/formatting.sh` script to format your changes before creating a PR.
+
+## Labeling your PR
+
+All PRs needs to be labeled as either "pr: non-breaking" or "pr: breaking".
+Label your PR as "pr: breaking" if you are introducing public API changes that breaks ABI compabibility,
+or you are introducing changes to the Slang language that will cause the compiler to error out on existing Slang code.
+It is rare for a PR to be a breaking change.
+
+## Testing
+
+Your PR should include a regression test for the bug you are fixing.
+Normally, these tests present as a `.slang` file under `tests/` directory.
+You will need to run your test with `slang-test tests/path/to/your-new-test.slang`. 

.github/workflows/copilot-setup-steps.yml

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+name: "Copilot Setup Steps"
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      pull-requests: write
+      checks: write
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: "recursive"
+          fetch-depth: "2"
+      - uses: ./.github/actions/format-setup
+      - name: setup environment
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libx11-dev