Keyboard shortcut to jump to the next function (DEFEDIT-1611)

In PICO-8 its Ctrl+Up and Ctrl+Down, super useful for navigating large files quickly. I have some big scripts and regularly need to move between a few functions, usually I end up doing a search for the function name and cycling through the call sites or something similar. Other navigation aids like a drop down list of all functions in the scope, split/multiview of the same file etc are always welcome.

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The issue does not name any files, tests, or editor entry points. Start by locating the editor's keyboard shortcut and code-navigation handling, then determine how functions are identified; done means a documented shortcut reliably moves between functions in a file and has coverage for the navigation behavior.

Written by the indexing model from the issue text.