add pre-commit configuration

This enables use of the `pre-commit` framework to run quick pre-commit
checks. In particular this allows to automatically fix:
* trailing white spaces
* absence or multiple newlines at the end of files
* QL code formatting
* file sync

More could be added in the future: anything that can be checked fast
can be added in the configuration (for example well-formedness of
`qldoc` files).

This is a purely opt-in feature. Instructions for enabling it and
possibly configuring its behaviour are in `pre-commit-hook-setup.md`.

.pre-commit-config.yaml

@@ -0,0 +1,21 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+exclude: /test/.*$(?<!\.ql)(?<!\.qll)(?<!\.qlref)
+repos:
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v3.2.0
+    hooks:
+    -   id: trailing-whitespace
+    -   id: end-of-file-fixer
+-   repo: local
+    hooks:
+    -   id: codeql-format
+        name: Fix QL file formatting
+        files: \.qll?$
+        language: system
+        entry: codeql query format --in-place
+    -   id: sync-files
+        name: Fix files required to be identical
+        language: system
+        entry: python3 config/sync-files.py --latest
+        pass_filenames: false