Explicitly add TS v4.9 to test matrix

We were testing this implicitly via `next` until 4.9 stabilized, at
which point `next` started targeting the 5.0 nightlies. With this
change, we now test against 4.8, 4.9, and 5.0 nightly. We'll need to
make a decision about what versions to support when we actually
"stabilize" the types: not merely publishing them, but opting consumers
into them by default. For now I'm leaving this as is because there's no
particular reason to bump past 4.8. (We might use `satisifies` in the
future, but nothing demands it at present.)

.github/workflows/ci.yml

@@ -52,7 +52,7 @@ jobs:
     needs: ['types']
     strategy:
       matrix:
-        ts-version: ['next']
+        ts-version: ['4.9', 'next']
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3