Move from npm to Yarn for markdown tests (#9312)

Yarn is better because...
* it validates package checksums
* it has emojis 🤟
* it is faster and employs better caching

.vsts-ci/misc-analysis.yml

@@ -37,6 +37,12 @@ jobs:
     condition: succeededOrFailed()
 
   - bash: |
+      curl -o- --progress-bar -L https://yarnpkg.com/install.sh | bash
+    displayName: Bootstrap Yarn
+    condition: succeededOrFailed()
+
+  - bash: |
+      # Installing globally with yarn is a pain, please don't try
       sudo npm install -g markdown-spellcheck@0.11.0
       # Sometimes this folder is left behind with root permissions and is needed by later npm installs which don't need sudo
       sudo rm -rf ~/.npm/_cacache

test/common/markdown/markdown.tests.ps1

@@ -17,11 +17,12 @@ Describe 'Common Tests - Validate Markdown Files' -Tag 'CI' {
         {
             $NpmInstalled = "Installed"
             Write-Verbose -Message "NPM is checking Gulp is installed. This may take a few moments." -Verbose
-            start-nativeExecution { npm install --silent }
-            start-nativeExecution { npm install 'gulp@4.0.0' --silent }
+            start-nativeExecution { yarn }
+            start-nativeExecution { yarn add gulp@4.0.0 }
             if(!(Get-Command -Name 'gulp' -ErrorAction SilentlyContinue))
             {
                 start-nativeExecution {
+                    # Installing globally with yarn is a pain, please don't try it
                     sudo npm install -g 'gulp@4.0.0' --silent
                     # Sometimes this folder is left behind with root permissions and is needed by later NPM installs which don't need sudo
                     sudo rm -rf ~/.npm/_cacache