chore: add limited public API for builtinRpcProcedures (#10499)

This is not a complete public API, just enough to avoid an `open
private` in ProofWidgets.

src/Lean/Server/Rpc/RequestHandling.lean

@@ -36,6 +36,10 @@ private unsafe def evalRpcProcedureUnsafe (env : Environment) (opts : Options) (
 private opaque evalRpcProcedure (env : Environment) (opts : Options) (procName : Name) :
     Except String RpcProcedure
 
+/-- Checks whether a builtin RPC procedure exists with the given name. -/
+def existsBuiltinRpcProcedure (method : Name) : IO Bool := do
+  return (← builtinRpcProcedures.get).contains method
+
 open RequestM in
 def handleRpcCall (p : Lsp.RpcCallParams) : RequestM (RequestTask Json) := do
   -- The imports are finished at this point, because the handleRequest function