Warn about missing file specified in `exports` `main` `module` `types`

[typed-sigterm]

Describe the feature

Just migrated from unbuild (with ext: 'js'), I almost forgot to change .[jt]s in the package to .m[jt]s.

If a non-existent file is referenced in the exports` `main` `module` `types of package.json, a warning after building may enhance DX.

Additional information

• Would you be willing to help implement this feature?