Update travis.yaml for pnpm

Author: dpvc

.travis.yml

@@ -1,17 +1,29 @@
+branches:
+  only:
+  - "/^\\d+\\.\\d+/"
+
 dist: jammy
+
 language: node_js
+
 node_js:
-- stable
+  - stable
+
 sudo: false
+
+before_install:
+  - npm install -g pnpm
+  - pnpm --version
+
+install:
+  - pnpm install
+
 script:
-- components/bin/version
-- npm config fix
-- npm install
-- npm run -s link:full
-- npm run -s build-all
-branches:
-  only:
-  - "/^\\d+\\.\\d+/"
+  - npm config fix
+  - components/bin/version
+  - pnpm run -s link:full
+  - pnpm run -s build-all
+
 deploy:
   provider: npm
   email: manager@mathjax.org