add pre-commit config

it's sane to have some pre-flight checks, pre-commit makes it easy.
ref: https://pre-commit.com/

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.0.1
+    hooks:
+      - id: check-merge-conflict
+      - id: check-toml