Generate dts by default if the input is TypeScript even if types field does not exist

[sapphi-red]

Currently tsdown does not generate dts if the package.json does not contain types field does not exist.
https://stackblitz.com/edit/github-d2t3ydlj?file=package.json

Instead of determining the behavior from the types field, I think it would be better to decide it based on the input.
If the input is typescript, I think you'd want to generate the dts as well.

types field is optional and when it's absent, TypeScript will infer the dts file path from main field / exports field. This is a valid setup and IMHO it's better than explicitly specifying the path because it's more simple.

[sxzz]

This behavior is currently based on unbuild and will be improved after issue #150 is resolved.

[sxzz]

Given the current situation, it’s still recommended to include the types field in your package for proper parsing by npmjs.com. For example, https://www.npmjs.com/package/@vitejs/plugin-vue does not indicate whether it provides TypeScript type definitions, if it did, a TS logo would appear next to its name.

Currently, we support detecting both the types field and exports.*.types. Another reason we don’t enable dts by default for TS projects is that generating dts for scenarios like CDN browsers, CLI, etc., is often unnecessary.

Therefore, I will open convert this to a discussion to explore possible changes.