Loads oj.config.{ts,js,mjs,json}.
.json is parsed directly. .ts/.js/.mjs have their TypeScript types
stripped with oxc, then evaluate in an embedded QuickJS engine (no Node)
with defineConfig and process.env shimmed, capturing the default
export and reading it back as JSON. Computed values, ternaries, and
process.env all work; the exported object must be JSON-serializable
(functions/plugins are out of scope until the plugin system).