feat: add supported version matrix action (#6)

supported-version/action.yml

@@ -0,0 +1,21 @@
+name: "Compute Supported Magento 2 Versions"
+author: "Graycore"
+description: "A Github Action that computes the Github Actions matrix for the currently supported versions of Magento 2"
+
+outputs:
+  matrix:
+    description: "The Github Actions matrix of software technologies required to run Magento."
+    value: ${{ steps.generate.outputs.matrix }}
+
+runs:
+  using: 'composite'
+  steps:
+  - run: |
+      content=`cat supported-version/supported.json | jq -c`
+      echo "::set-output name=matrix::$content"
+    shell: bash
+    id: generate
+    
+branding:
+  icon: "code"
+  color: "green"