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Except for core functionality, everything in JetBrains' IDEs is written as plugins (grammar checking, themes, support for Go/PHP/Ruby/Python/Android/framework of the month, the list goes on). You just don't notice it because of good integration with the rest of the editor (unlike how it's done in Eclipse).

Those can be (and are) developed by anybody. For example, IDEA has had absolutely the best Rust story for many years, and the Rust plugin became official only recently. It was created and developed for years before that by a single developer not affiliated with JB (and still blew all other editors out of the water).