feat: use composite action instead of node

Using composite with two actions:
- rlespinasse/slugify-value for the SLUG features
- rlespinasse/shortify-git-revision for the SHORT feature

Since the SHORT sha is now based on a shorted git revision,
and no more on a fix substring.

BREAKING CHANGE: use different backend for the action

action.yml

@@ -1,9 +1,62 @@
 name: "GitHub Slug Action"
 description: "GitHub Action to expose slug value of environment variables inside your GitHub workflow"
-author: 'rlespinasse'
-runs:
-  using: 'node12'
-  main: 'dist/index.js'
+author: "Romain Lespinasse"
 branding:
   icon: "minimize"
   color: "blue"
+runs:
+  using: "composite"
+  steps:
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_REPOSITORY
+        value: ${{ github.repository }}
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_REF
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_HEAD_REF
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_BASE_REF
+
+    # Specific values
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_EVENT_REF
+        value: ${{ github.event.ref }}
+
+    # Calculated values
+    - id: get-github-ref-name
+      run: echo "::set-output name=github-ref-name::$(echo "${{ github.head_ref || github.ref }}" | cut -d/ -f3)"
+      shell: bash
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_REF_NAME
+        value: ${{ steps.get-github-ref-name.outputs.github-ref-name }}
+    - id: get-github-repository-owner-part
+      run: echo "::set-output name=github-repository-owner-part::$(echo $GITHUB_REPOSITORY | cut -d/ -f1)"
+      shell: bash
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_REPOSITORY_OWNER_PART
+        value: ${{ steps.get-github-repository-owner-part.outputs.github-repository-owner-part }}
+    - id: get-github-repository-name-part
+      run: echo "::set-output name=github-repository-name-part::$(echo $GITHUB_REPOSITORY | cut -d/ -f2)"
+      shell: bash
+    - uses: rlespinasse/slugify-value@v1.1.0
+      with:
+        key: GITHUB_REPOSITORY_NAME_PART
+        value: ${{ steps.get-github-repository-name-part.outputs.github-repository-name-part }}
+
+    # Short
+    - uses: rlespinasse/shortify-git-revision@v1.2.0
+      with:
+        name: GITHUB_SHA
+        continue-on-error: true
+    - uses: rlespinasse/shortify-git-revision@v1.2.0
+      with:
+        name: GITHUB_EVENT_PULL_REQUEST_HEAD_SHA
+        revision: ${{ github.event.pull_request.head.sha }}
+        continue-on-error: true