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Use the New File → Module Development → File Typewizard. A MIME type must be specified. This MIME type will be the key under which other services will be looked up.
See the File Type Integration Tutorial for more details on how to add File Type recognition support. A DataObject file will be created with a number of annotations.
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You may want to create a new "Project Type" for your specific language. More explicitly, when you click on File → New Project, you can customize the New Project wizard dialog to create a new project for your language. For instance, "Rust" projects usually have a folder structure defined by the "cargo" tool. rust/rust.project.api module allows the user to create a new cargo project.
The NetBeans Project Type Tutorial is a good starting point.
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NetBeans IDE provides the Options window (menu Tools → Options or NetBeans → Preferences on MacOS) that allows the user to customize it. You can provide any customizations for your language support in the Options window, too. E.g. you could allow the user to provide the path to the Kotlin compiler. See e.g. rust/rust.cargo module that allows the user to provide the path to cargo for Rust projects.
The NetBeans Options Window Module Tutorial explains how you could do that.
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Refactorings like e.g. rename, move are supported by all major IDEs.
10. Error squiggles and apply suggestions from errors
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