CI - Only allow release/* branches to merge into master (#124)

## Description of Changes
Redo these PRs:
-
clockworklabs/com.clockworklabs.spacetimedbsdk-archive#56
-
clockworklabs/com.clockworklabs.spacetimedbsdk-archive#59
-
clockworklabs/com.clockworklabs.spacetimedbsdk-archive#61

I'm not sure how these repos were merged in
#117, but it
appears that this wasn't carried over. (Either way we need to make this
a required check).

## API
No


## Requires SpacetimeDB PRs
*List any PRs here that are required for this SDK change to work*

Co-authored-by: Zeke Foppa <[email protected]>

Author: bfops

.github/workflows/check-pr-base.yml

@@ -0,0 +1,22 @@
+name: Git tree checks
+
+on:
+  pull_request:
+    types: [opened, edited, reopened, synchronize]
+  merge_group:
+permissions: read-all
+
+jobs:
+  check_base_ref:
+    name: Only release branches may merge into master
+    runs-on: ubuntu-latest
+    steps:
+      - id: not_based_on_master
+        if: |
+          github.event_name == 'pull_request' &&
+          github.event.pull_request.base.ref == 'master' &&
+          ! startsWith(github.event.pull_request.head.ref, 'release/')
+        run: |
+          echo 'Only `release/*` branches are allowed to merge into `master`.'
+          echo 'Maybe your PR should be merging into `staging`?'
+          exit 1