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/actions/composite/action.yml

@@ -0,0 +1,22 @@
+name: 'Test Composite Action'
+description: 'Test Composite Action'
+inputs:
+  input:
+    description: 'String to echo'
+    required: true
+    default: 'Hello World'
+outputs:
+  output:
+    description: 'Output string'
+    value: ${{ steps.set-output.outputs.string }}
+runs:
+  using: "composite"
+  steps:
+    - run: echo Hello ${{ inputs.input }}.
+      shell: bash
+    - id: set-output
+      run: echo "::set-output name=string::output string"
+      shell: bash
+    - run: |
+        ${{ github.action_path }}/script.sh
+      shell: bash

.github/actions/composite/script.sh

@@ -0,0 +1 @@
+echo "Output from script"