string - leftpad, rightpad would be usefull

I would need additional string filter for padding (leftpad, rightpad)

What do you think?

Look at the existing string filters in the source code, likely in a file like `src/string.js`. Understand how filters are registered in AngularJS. Implement leftpad and rightpad functions that take a string, target length, and optional pad character. Write tests for the new filters, following the project's testing pattern. Check if there are any existing examples of similar filters to ensure consistency.

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