chore: simplify build with npm workspaces

[refactorthis]

Description

• simplified build process with npm workspaces
• updated GH workflows to use new npm scripts
• simplified caching in GH workflows

Notes:

• I suggest that we move the packages into packages/* to fit monorepo convention, however this is a large change, so I have not made it here.
• I notice other utility packages in the packages folder were not being built as a part of the install script, or GH workflow, so have not added them to monorepo, I think they should probably be in there - code owners to clarify.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created

Testing

GH workflows were tested with act, however not end-to-end on GH. Please pay particular attention to reviewing these files.