Add preview deployments

.github/workflows/preview_build.yml

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+# This action builds and deploys egui_demo_app on each pull request created
+# Security notes:
+# This action is split in two workflows, preview_build and preview_deploy.
+# `preview_build` runs on pull_request, so it won't have any access to the repositories secrets, so it is safe to execute
+# untrusted code.
+# `preview_deploy` has access to the repositories secrets (so it can push to the pr preview repo) but won't run
+# any untrusted code (it will just extract the build artifact and push it to the pages branch where it will
+# automatically be deployed).
+
+on:
+  - pull_request
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: rustup toolchain install stable --profile minimal --target wasm32-unknown-unknown
+      - uses: Swatinem/rust-cache@v2
+        with:
+          prefix-key: "pr-preview-"
+
+      - name: "Install wasmopt / binaryen"
+        run: |
+          sudo apt-get update && sudo apt-get install binaryen
+
+      - run: |
+          scripts/build_demo_web.sh --release
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: web_demo
+          path: web_demo
+
+      - name: Generate meta.json
+        env:
+          PR_NUMBER: ${{ github.event.number }}
+          PR_BRANCH: ${{ github.head_ref }}
+        run: |
+          echo "{\"pr_number\": \"$PR_NUMBER\", \"pr_branch\": \"$PR_BRANCH\"}" > meta.json
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: meta.json
+          path: meta.json

.github/workflows/preview_cleanup.yml

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+on:
+  pull_request_target:
+    types:
+      - closed
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - run: mkdir -p empty_dir
+      - name: Url slug variable
+        run: |
+          echo "URL_SLUG=${{ github.event.pull_request.number }}-${{ github.event.pull_request.head.ref }}" >> $GITHUB_ENV
+      - name: Deploy
+        uses: JamesIves/github-pages-deploy-action@v4
+        if: secrets.DEPLOY_KEY
+        with:
+          folder: empty_dir
+          repository-name: egui-pr-preview/pr
+          branch: 'main'
+          clean: true
+          target-folder: ${{ env.URL_SLUG }}
+          ssh-key: ${{ secrets.DEPLOY_KEY }}
+          commit-message: "Remove preview for PR ${{ env.URL_SLUG }}"

.github/workflows/preview_deploy.yml

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+on:
+  workflow_run:
+    workflows:
+      - preview_build
+    types:
+      - completed
+
+
+permissions:
+  pull-requests: write
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download Artifact
+        uses: actions/download-artifact@v4
+        with:
+          run-id: '${{ github.event.workflow_run.id }}'
+
+      - name: Parse meta.json
+        run: |
+          echo "PR_NUMBER=$(jq -r .pr_number meta.json)" >> $GITHUB_ENV
+          echo "PR_BRANCH=$(jq -r .pr_branch meta.json)" >> $GITHUB_ENV
+
+      - name: Url slug variable
+        run: |
+          echo "URL_SLUG=${{ env.PR_NUMBER }}-${{ env.PR_BRANCH }}" >> $GITHUB_ENV
+
+      - name: Deploy
+        uses: JamesIves/github-pages-deploy-action@v4
+        if: secrets.DEPLOY_KEY
+        with:
+          folder: web_demo
+          repository-name: egui-pr-preview/pr
+          branch: 'main'
+          clean: true
+          target-folder: ${{ env.URL_SLUG }}
+          ssh-key: ${{ secrets.DEPLOY_KEY }}
+          commit-message: "Update preview for PR ${{ env.URL_SLUG }}"
+          force: false
+
+      - name: Comment PR
+        uses: thollander/actions-comment-pull-request@v2
+        with:
+          message: |
+            Preview available at https://egui-pr-preview.github.io/pr/${{ env.URL_SLUG }}