Replace subtraction of Instants and Durations with saturating subtractions (#12549)

# Description
Duration can not be negative, and an underflow causes a panic.

This should fix #12539 as from what I can tell that bug was caused in
`nu-explore:📟:events` from subtracting durations, but I figured
this might be more widespread, and saturating to zero generally makes
sense.

I also added the relevant clippy lint to try to prevent this from
happening in the future. I can't think of a reason we would ever want to
subtract durations without checking first.

cc @fdncred

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-explore/src/pager/events.rs

@@ -47,7 +47,7 @@ impl UIEvents {
                 }
 
                 let time_spent = now.elapsed();
-                let rest = self.tick_rate - time_spent;
+                let rest = self.tick_rate.saturating_sub(time_spent);
 
                 Self { tick_rate: rest }.next()
             }