Harper could have support for GNOME Builder

GNOME Builder is an IDE for GNOME and GTK

It could have a Harper extension either via [its extension API](https://builder.readthedocs.io/plugins/creating.html) or `harper-ls`

It wouldn't have to be part of the Harper project. It could be independent or part of the GNOME Builder project.

This would be a good first issue for somebody who already knows GNOME Builder.

Start with the GNOME Builder extension API documentation linked in the issue, then compare that integration path with the existing harper-ls entry point. Done means GNOME Builder can use Harper for its intended language-checking workflow, either through an extension or harper-ls, with the project boundary documented.

Written by the indexing model from the issue text.