Add keyboard shortcuts to center scrolling around current line

[notpeter]

• MacOS: Center the cursor in the visible area. ctrl-l (matches MacOS)
• Linux JetBrains: Scroll so cursor is at the Middle ctrl-m
• editor::NextScreen is not longer bound in any keymap by default (was ctrl-l on MacOS)

Fixes #5247

Release Notes:

• Added "Center the cursor in the visible area." to match MacOS ctrl-l; Added ctrl-m for JetBrain on Linux for the same (#5247).