Update TS test matrix to cover 4.6

Author: markerikson

.github/workflows/tests.yml

@@ -96,7 +96,7 @@ jobs:
       fail-fast: false
       matrix:
         node: ['14.x']
-        ts: ['4.1', '4.2', '4.3', '4.4', '4.5', 'next']
+        ts: ['4.1', '4.2', '4.3', '4.4', '4.5', '4.6.1-rc', 'next']
     steps:
       - name: Checkout repo
         uses: actions/checkout@v2