This PR merges the schemas/ directory (#145) into schema/ (from #146).

Additionally, it makes some extra changes:

• Because when we do schema validation we have to load all schemas into memory, I thought it would make sense that we also generate the final schema.json at that point. So here I changed validate.mjs into build.mjs which now does both validation and generation of the final schema.json.
• The final schema.json file is generated into the schema/ directory itself, but hidden from git
• Then I changed webpack so that it just calls build.mjs and then reads the generated schema.json, instead of webpack itself having logic for generating the final schema.json.
• This makes it possible to centralize the logic of building the schema into one place (schema/build.mjs), which can also be ran from the CLI.
• I also moved the individual schemas into a schema/subjects directory, so that they are separate from all other files
• I also added a build:schema script to package.json