Add prepare script to build package on install

When installing from git, npm runs the 'prepare' script automatically.
This ensures the package is built without needing to commit dist/ files.

This allows consuming repos to install the package via:
npm install @salesforce/vscode-extension-ci@github:salesforcecli/github-workflows#feat/add-vscode-extension-ci

Author: madhur310

packages/vscode-extension-ci/package.json

@@ -13,6 +13,7 @@
     "clean": "rm -rf dist",
     "test": "echo \"Tests not yet implemented\"",
     "lint": "eslint src --ext .ts",
+    "prepare": "npm run build",
     "prepublishOnly": "npm run build"
   },
   "files": [