ci: Add linter to make sure we use Conventional Commits

This will make it easier for us to write Changelogs for example.
See https://www.conventionalcommits.org/en/v1.0.0.
Also mentioned in the CONTRIBUTING.md

Signed-off-by: Michael Vetter <jubalh@iodoru.org>

.commitlintrc.json

@@ -0,0 +1,23 @@
+{
+  "extends": ["@commitlint/config-conventional"],
+  "rules": {
+    "type-enum": [
+      2,
+      "always",
+      [
+        "feat",
+        "fix",
+        "docs",
+        "style",
+        "refactor",
+        "perf",
+        "tests",
+        "build",
+        "ci",
+        "chore",
+        "cleanup"
+      ]
+    ],
+    "subject-case": [0]
+  }
+}