One of my favourite editors from the 1990's was FrexxEd, co-authored by the author of Curl, whose main event loop processed Arexx commands and events for every

One of my favourite editors from the 1990's was FrexxEd, co-authored by the author of Curl, whose main event loop processed Arexx commands and events for every internal command, so you could rebind every event to a script in their own scripting language (FPL; C-like), and access every internal function from it. (It incidentally also came with FPL scripts that provided some degree of syntax highlighting for a few languages, and you could add more, though not as expansive as most modern editors). Running a heavily scriptable editor with a GUI was entirely viable on a 7.16MHz Amiga with 1MB RAM, and more responsive than many modern editors. Integration with other tooling, like RCS, compilers, or linkers was a given for Amiga apps at that time. (FrexxEd was also notable for exposing internal buffers as files in a special filesystem, so you could e.g. lint or compile or call your revision control - limited as they were - directly on the buffers without any custom integration)