github: Update publishing workflow & make it single-step

Changelog in Marketplace entry is sourced automatically from Changelog.md
We can continue updating that changelog manually and ensure it's not forgotten
via a custom input where a maintainer has to confirm this.

We can also continue copy-pasting the relevant part of changelog
into GitHub Release notes.

We can use Linux instead of Windows since Windows is not a requirement for vsce.

Lastly we should not need to continue publishing the vsix to GitHub
as this encourages a bad practice of users installing released extension
outside of Marketplace.

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
       - name: setup node
         uses: actions/setup-node@v1
         with:
-          node-version: '14.x'
+          node-version: '12.x'
 
       - name: clean install dependencies
         run: npm ci    

.github/workflows/publish.yml

@@ -1,23 +1,59 @@
 name: Publish release
-on:
-  release:
-    types:
-      - published
+
+on: 
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'Version to publish & tag & push (without "v" prefix)'
+        required: true
+      changelog_updated:
+        description: 'Did you update the changelog? ("yes")'
+        required: true
 
 jobs:
   publish:
-    name: Publish to Marketplace
-    runs-on: windows-latest
+    name: Publish to VS Code Marketplace
+    runs-on: ubuntu-latest
     steps:
+      - name: Ensure changelog is updated
+        if: ${{ github.event.inputs.changelog_updated != 'yes' }}
+        run: |
+          echo "Response: ${{ github.event.inputs.changelog_updated }} (expected yes)"
+          exit 1
       - name: Check out repository
         uses: actions/checkout@v2
-      - name: Set up Node
+      - name: Read Node.js version
+        id: nodejs-version
+        run: |
+          content=`cat ./.nvmrc`
+          echo "::set-output name=content::$content"
+      - name: Set up Node.js
         uses: actions/setup-node@v1
         with:
-          node-version: '14.x'
-      - name: Clean install dependencies
+          # See https://github.com/actions/setup-node/issues/32
+          node-version: ${{ steps.nodejs-version.outputs.content }}
+      - name: Install dependencies
         run: npm ci
+      - name: Install Terraform
+        uses: hashicorp/setup-terraform@v1
+        with:
+          terraform_wrapper: false
+          terraform_version: "~1.0"
+      - name: Set up Xvfb (Ubuntu)
+        run: |
+          /usr/bin/Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
+      - name: npm test
+        run: npm test
+      - name: Configure git author
+        run: |
+          git config user.email "hashibot-feedback@hashicorp.com"
+          git config user.name "HashiBot"
       - name: vsce publish
-        run: vsce publish
+        run: npx -- vsce publish ${{ github.event.inputs.version }}
         env:
           VSCE_PAT: ${{ secrets.VSCE_PAT }}
+      - name: git push
+        run: |
+          $defaultBranch = git branch --show-current
+          git push origin $defaultBranch
+          git push origin v${{ github.event.inputs.version }}