fix!: allow to remove shortcuts

[twitwi]

defineShortcutsSetup are supposed to receive the set of shortcuts and return the new set of shortcuts (based on the received one, possibly removing some). So we have a chain of setups.

The current code does a union of setups, merging all results, breaking the previous semantics (which is more useful and which is reflected by the documentation).

This patch brings back the new behavior, using the new, clean setup syntax.

[netlify]

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