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Go is a language like that, with e.g. the gorename tool for refactoring.

This is basically the idea behind the Language Server Protocol. The language toolchain provides a server and the IDE is a client.

https://microsoft.github.io/language-server-protocol/

The dividing line between what's a language and what's the IDE is fairly arbitrary

How do you figure? The language defines the runtime semantics in a way no IDE or refactoring tool does. It's a very bright line, unless you're conflating the distribution package of the language with the language itself.

A language and an IDE are completely separate things.