Add support for composite actions (#514)

* Add support for composite actions

* Fix to make more complex composite actions work

* Fix to make more complex composite actions work

* Let's validate the steps in the composite steps to fail on uses and run's without shell, like the real world

* Add support for composite actions

* Add workflow to test composite actions

* Log instead of panicing when output is mismatched

* Merge maps so environment variables are not lost

* Remove Debug

* Correect merge error

* Remove invalid composite tests.

* Fix composite test

Co-authored-by: Casey Lee <cplee@nektos.com>
Co-authored-by: monkers <mikem@msquaredconsulting.co.uk>
Co-authored-by: Mike Moncrieffe <69815687+mikemonkers@users.noreply.github.com>

.github/workflows/test-composite.yml

@@ -0,0 +1,25 @@
+name: "Test composite actions"
+on: push
+
+env:
+  ACT: true
+
+jobs:
+  test-composite-actions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: "✅ I should echo default 'Hello World'"
+        id: step0
+        uses: ./.github/actions/composite
+
+      - name: "Check action output"
+        if: steps.step0.outputs.output != 'output string'
+        run: echo "expected output to be 'output string' but it wasn't"
+
+      - name: "✅ I should echo 'Test string'"
+        id: step1
+        uses: ./.github/actions/composite
+        with:
+          input: 'Test string'