Update to PNPM 10

[rmunn]

PNPM 10 is now in RC. One of its major features is that "Lifecycle scripts of dependencies are not executed during installation by default! This is a breaking change aimed at increasing security."

I've read of several supply-chain attacks in NPM recently, many of which were aimed at installing bitcoin miners on target computers, so this strikes me as a good idea in general. But we'll need to edit package.json to include a pnpm.onlyBuiltDependencies field listing the packages that should be allowed to run their lifecycle scripts (e.g. playwright wants to download updated browser versions).

This is probably low priority since PNPM 9 is working just fine so far, but in the long run I think the extra security that will come with PNPM 10 means we'll want to upgrade once we've checked which packages should be allowed to run scripts.