Don't apply Ctrl markdown shortcuts on Mac
Background
In !40328 (merged), a few keyboard shortcuts were added to the markdown editor for common formatting options.
In order to make these shortcuts cross-platform, each formatting option was bound to two shortcuts: Ctrl+<key> and ⌘+<key>.
However, on Mac, this collided with existing readline shortcuts: #26989 (comment 412058803)
What does this MR do?
Instead of binding both Ctrl+<key> and ⌘+<key>, this MR updates the shortcuts to be defined with the generic mod
key provided by Mousetrap.js so that only the appropriate shortcut for the current platform is applied.
So, for example, on Mac, ⌘+b is bound to "bold", but Ctrl+b is unaffected.
On non-Mac platforms, the reverse is true.
Documentation updates
No documentation updates are necessary because the docs already imply that this is how the shortcuts work.