Allow library building from source when including in `package.json`

[RinkeHoekstra]

The current package.json does not have a prepare step, which means that it is hard for development projects to include a specific branch or commit to their dependencies (as no libraries will be built automatically).

I propose to include:

"prepare": "npm run build && npm run build:types && npm run build:browser && npm run build:esm",

This allows projects to include rdflib as follows in their package.json dependencies:

"rdflib": "github:linkeddata/rdflib.js#COMMITHASH",

As per https://docs.npmjs.com/cli/v10/configuring-npm/package-json#git-urls-as-dependencies