Add @lightningtv/source export condition

[thetarnav]

A custom export condition allows for the source to be used by typescript/vite directly, without rebuilding, with minimal configuration, while not affecting those who do not enable it.

All thats necessary to use it:

// vite config
export default defineConfig({
  resolve: {
    conditions: ["@lightningtv/source"],
  },
});

// ts config
{
  "compilerOptions": {
    "customConditions": ["@lightningtv/source"],
  }
}

It's taken from this article and it works pretty well. I use it in solid-primitives and solid-devtools monorepos, but it should also work for linking packages with pnpm link.

I also removed the "main", "types" and "browser" fields because they are used by old cjs tools, which this package doesn't support anyway.