Add CI support for merge queues.

This should let us use merge queues, which will save some of the pain of
waiting for CI to run before being able to press buttons.

.github/workflows/main.yml

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 1a024d9..25cddb5 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -5,6 +5,7 @@
     branches: [ main ]
   pull_request:
     branches: [ main ]
+  merge_group:
   workflow_dispatch:
 
 jobs: