feat: Enable to export named component by adding component-export attribute

[usualoma]

fixes #156

Use the component-export mentioned in isseu to wrap an export matching the following name as an island component.

https://github.com/honojs/honox/compare/main...usualoma:honox:feat/export-named-component?expand=1#diff-dbf4bfe9bdc2ab09701d7f99d62aa69e45740919b350c8166aebe64daa3fdd54R39-R41