BLD: require pnpm, remove support for npm

[shilorigins]

Description

• add packageManager with pnpm version to package.json
• remove package-lock.json
• change npm to pnpm throughout README
• use pnpm in linting workflow

Motivation

Following eed-web-application/elog-plus-frontend#187, this PR limits support to one package manager and pnpm is considered to be slightly faster and have a slightly smaller footprint.

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions