Implements RFC #2963. Leftover tasks: - [x] Provide companion PR to vscode-lean4 (leanprover/vscode-lean4#376) - [x] Companion PR to std4 (leanprover/std4#467) - [x] Companion PR to ProofWidgets4 (leanprover-community/ProofWidgets4#36) - [X] Companion commit to mathlib4 (0f4660f655) - [ ] ~~Update the manual chapter~~ (will do in a follow-up)
0f4660f655
UserWidgetDefinition
To satisfy https://github.com/leanprover/lean4/pull/1238#discussion_r908839474