This lays some groundwork for future PRs that will run builds in
parallel, prefixing each line output with the appropriate slug.

The helpers in `helpers/tasks/installProjectTask.js` weren't all that
helpful anymore. Instead we provide some lower-level functions in
`helpers/install.js` that can be built upon for the slightly different
future requirements of install versus build.

Install no longer confusingly reports that it's installing pnpm
dependencies for every package being installed, instead doing a single
`pnpm install` task at the top level.

Install no longer behaves weirdly when passed both `--root` and `--all`.

Build gets an `--all` option (because it was easy to add), and an option
to skip the `pnpm install`.

Build also handles its own output in verbose mode (rather than using
listr-verbose-renderer) in preparation for parallel builds.

Due to the abstracting of `jetpack install --all` and `jetpack build
packages`, those commands now also accept multiple packages on the
command line.

`jetpack build --timing` will now output timing information for each
completed command. With `--verbose`, it will also prefix each line of
output with the time since the start of the task.