Add SemVer compatibility checks to CI

We recently introduced release branches that need to remain backwards
compatible. However, even small changes to item visibility during
backporting fixes might introduce SemVer violations (see
https://doc.rust-lang.org/cargo/reference/semver.html#change-categories
for a list of changs that would be considered major/minor).

To make sure we don't accidentally introduce such changes in the
backports, we here add a new `semver-checks` CI job that utilizes
`cargo-semver-checks`
(https://github.com/obi1kenobi/cargo-semver-checks), and have it run on
any push or pull requests towards anything else but `main`/`master`
(i.e., all feature branches to come).

.github/workflows/semver.yml

@@ -0,0 +1,20 @@
+name: SemVer checks
+on:
+  push:
+    branches-ignore:
+      - master
+      - main
+  pull_request:
+    branches-ignore:
+      - master
+      - main
+
+jobs:
+  semver-checks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v4
+      - name: Check SemVer with default features
+        uses: obi1kenobi/cargo-semver-checks-action@v2
+        feature-group: default-features