Add npm install in order to add to bom

[darinpope]

In trying to add this plugin to bom, I was unable to run:

mvn clean verify

locally to do testing since I did not have npm installed:

[INFO] --- frontend:1.15.0:yarn (yarn mvnbuild) @ customizable-header ---
[INFO] Running 'yarn run mvnbuild' in /Users/dpope/github-forks/customizable-header-plugin
[INFO] yarn run v1.22.19
[INFO] $ npm run build
[INFO] /bin/sh: npm: command not found
[INFO] error Command failed with exit code 127.
[INFO] error Command failed with exit code 127.info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

There may be a better way to solve this issue, but this was the simplest way for me.

Once this issue is solved (in whatever way you prefer), I can add this plugin to bom.

Testing done

mvn clean verify succeeds

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue