Clean up npm publish output and add exports map

[domachine]

The published package currently ships no TypeScript types and has no exports field, leaving the API surface undefined. This issue tracks three small, low-risk changes to package.json:

1. Add prepublishOnly script — runs tsc -b . before every npm publish to ensure .d.ts files are always generated and up-to-date.

2. Add files array — add relevant modules and build/*.d.ts so the public modules and the generated type declarations are included in the package. Dev artefacts (scripts, tests, csaf_2_1, etc.) are explicitly excluded.

3. Add exports map — declare the public sub-path entry points (e.g. "./validate.js", "./basic.js") each with a types condition pointing to build/*.d.ts and an import condition pointing to the source .js. This formalises the API surface and prevents consumers from reaching into lib or schemas directly.