feat: introduce pre-commit framework

This replaces the custom clang-format script with the pre-commit framework,
making it easier to add checks in the future and simplifying GitHub
integration.

The GH workflow is adapted to run checks now as well.

To use it locally, all you need to do is install pre-commit locally, typically
via:
```
pip3 install --user pre-commit
```

You can then run `pre-commit run --all` to run it locally. Note that this does _not_ install
a Git hook. If you want that as well, run `pre-commit install`.

For more details on pre-commit and other installation options, see
https://pre-commit.com and https://pre-commit.com/#install

.github/workflows/pre-commit.yml

@@ -0,0 +1,26 @@
+name: Pre-commit checks
+
+on:
+  # Run workflow for PRs.
+  pull_request:
+
+  # Whenever we have a new commit on main, run the workflow for that.
+  push:
+    branches: [main]
+
+jobs:
+  pre-commit:
+    name: Pre-commit checks
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          ref: ${{github.event.pull_request.head.sha}}
+          repository: ${{github.event.pull_request.head.repo.full_name}}
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.10"
+      - name: Run pre-commit checks on PR
+        uses: pre-commit/[email protected]

.pre-commit-config.yaml

@@ -0,0 +1,9 @@
+exclude: 'test/(catch.hpp|tests-main.cpp)'
+
+repos:
+-   repo: https://github.com/pre-commit/mirrors-clang-format
+    rev: v18.1.2
+    hooks:
+    -   id: clang-format
+        exclude_types: [json]
+