Switch to pnpm (and update some dependencies)

[Gold856]

Description

pnpm is much faster than npm for installing dependencies, and it prevents accidently depending on dependencies not listed in package.json. Some dependencies were also scoped to devDependencies to just to neaten things up. Caching has also been enabled in CI, so commits that are pushed in quick succession finish building faster.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2024.3.1
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[mcm001]

Please also update building-photon.md if we are dropping npm entirely

[Gold856]

Already done.