New rule proposal: no-floating-error

[remorses]

Can I open a PR to add a no-floating-error rule? It would flag when a call expression returns a type containing Error (or a subclass) and the result is discarded as an expression statement.

This enforces the "errors as values" pattern from errore, where functions return Error | T unions instead of throwing. Callers must check instanceof Error before proceeding, and discarding the result means the caller forgot to handle the error.

// bad: error return discarded
declare function getUser(id: string): Error | User
getUser(id)

// good: error handled
const user = getUser(id)
if (user instanceof Error) return user

// good: explicitly ignored
void getUser(id)

I have a working implementation with tests already.

[remorses]

Another idea: a broader no-floating-result rule that requires the return value of any non-void function to be used or explicitly discarded with void. This would catch more cases but would be heavier since it flags every unused return value, not just Error types.