Fix panic due to bad iter.Seq implementation (#4)

The following panic was occasionally crashing users of this library:
```
panic: runtime error: range function continued iteration after function for loop body returned false

goroutine 372057 [running]:
github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).notifySubscribers-range1({0x17a9ae0?, 0xc019746070?, 0x23db800?})
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:319 +0x1c5
github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).iterateSubscribers.func1-range1({0xc06428dc68, 0x472988, 0x23db800?})
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:406 +0x143
github.com/linkedin/diderot/internal/cache.(*SubscriberSet[...]).Iterator.func2.1({0x1265ec0?, 0xc094fad000})
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/subscriber_set.go:127 +0x90
sync.(*Map).Range(0xc014669040?, 0xc06428dd28)
        /export/home/tester/.go/1.23.0/src/sync/map.go:501 +0x1f8
github.com/linkedin/diderot/internal/cache.(*SubscriberSet[...]).Iterator.func2()
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/subscriber_set.go:122 +0x45
github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).iterateSubscribers.func1()
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:398 +0x139
github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).notifySubscribers(0x17e17c0, {0xc01cc25c00, {0x24c6ac40, 0xede925733, 0x23db800}, {0xc1b835ece4e2c066, 0x70e6bc3a557, 0x23db800}, 0x2, {0xc01ce9f8c0, ...}}, ...)
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:319 +0x1be
github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).startNotificationLoop.func1()
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:282 +0x2b5
created by github.com/linkedin/diderot/internal/cache.(*WatchableValue[...]).startNotificationLoop in goroutine 117232
        /export/home/tester/.cache/go/pkg/mod/github.com/linkedin/diderot@v0.0.2/internal/cache/watchable_value.go:261 +0x87
```
This is due to the fact that the nested for loop continues after `yield`
returns false. Using `iter.Seq` makes for nice syntactic sugar, but can
be occasionally tricky to write.

internal/cache/watchable_value.go

@@ -404,7 +404,7 @@ func (v *WatchableValue[T]) iterateSubscribers(
 				}
 
 				if !yield(handler, subscribedAt) {
-					break
+					return
 				}
 			}
 		}