Add Node.js to dev tools

What does this MR do?

Adds nodejs (and with it npm) to the repository's dev tools, managed by mise.

The nodejs version is pinned to the most recent LTS release, which is 24.15.0.

Why was this MR needed?

markdownlint-cli2 fails to install if there is no npm installation locally.

This is because markdownlint-cli2 is an npm package, but npm is not currently managed as a dev tool in the project, and so its presence locally is not guaranteed.

Below are some log excerpts from running mise install in the project root, showing the current issues:

mise WARN  npm may be required but was not found.

To use npm packages with mise, you need to install Node.js first:
mise use node@latest

Alternatively, you can use bun or pnpm instead of npm by setting:
mise settings npm.package_manager=bun

markdownlint-cli2@0.22.1 install                                                                                                                               mise ERROR Failed to install tools: npm:markdownlint-cli2@0.22.1

npm:markdownlint-cli2@0.22.1: failed to execute command: npm install -g markdownlint-cli2@0.22.1 --prefix /Users/danielkeenan/.local/share/mise/installs/markdownlint-cli2/0.22.1: No such file or directory (os error 2)

What's the best way to test this MR?

In a mise-activated shell, run:

mise install
node --version
npm --version
markdownlint-cli2 --version

All three should print version strings without error.

What are the relevant issue numbers?

N/A