Add workflow "PR size labeler" to label PRs based on the number of changed lines

This should help reviewers to determine which PRs to review.

.github/changed-lines-count-labeler.yml

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+# Add 'size/small' label to any changes with less than 50 lines
+size/small:
+  max: 49
+
+# Add 'size/medium' label to any changes between 50 and 249 lines
+size/medium:
+  min: 50
+  max: 249
+
+# Add 'size/large' label to any changes between 250 and 749 lines
+size/large:
+  min: 250
+  max: 749
+
+# Add 'size/giant' label to any changes for more than 749 lines
+size/giant:
+  min: 750

.github/workflows/pr-labeler.yml

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+name: "PR size labeler"
+on: [pull_request]
+
+jobs:
+  changed-lines-count-labeler:
+    runs-on: ubuntu-latest
+    name: Automatically labelling pull requests based on the changed lines count
+    steps:
+    - name: Set a label
+      uses: vkirilichev/changed-lines-count-labeler@v0.2
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        configuration-path: .github/changed-lines-count-labeler.yml