chore(deps): bump typescript to 4

Bumped `typescript` up to v4.x as well as a bunch of other dependencies,
including `jest` which had a moderate severity from it's `node-notifier`
dependency. Also added Node.js v15.x to the automated testing.

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ['10', '12', '14']
+        node-version: ['10', '12', '14', '15']
     steps:
     - name: Checkout Code
       uses: actions/checkout@v2
@@ -25,7 +25,7 @@ jobs:
     - name: Run Tests
       run: npm run test:coverage
     - name: Upload Coverage
-      if: ${{ matrix.node-version == '14' }}
+      if: ${{ matrix.node-version == '15' }}
       uses: codecov/codecov-action@v1
       with:
         file: ./coverage/lcov.info