Add Squiggle Lang Jest GitHub Action

.github/workflows/lang-jest.yml

@@ -0,0 +1,16 @@
+name: Squiggle Lang Jest Tests
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    run:
+      shell: bash
+      working-directory: packages/squiggle-lang
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install Packages
+      run: yarn 
+    - name: Run tests
+      run: yarn test