Fix for Issue #775: Prevent Duplicate Import Statements in Generated Templates

[akdotio]

This PR addresses issue #775, where duplicate import statements in generated templates caused go build to fail.

Changes Made:

• Updated the template generator to check for existing import statements before adding new ones.
• Added tests to ensure that import statements are only included once, even if explicitly imported in the template file.

With these changes, the generator will no longer produce duplicate import statements, ensuring successful builds.

[a-h]

Thanks a lot for for putting this together, but we realised that we needed to sort out the organisation of imports too, so we've implemented a fix for the imports more generally at https://github.com/a-h/templ/pull/793

It makes it so that there's only two imports - templ, and templruntime, so the generated code won't have magic imports that you can't see like io etc. Additionally, any missing imports will be added automatically.