v2: Stopwatch doesn't default to 1 second as documented

**Describe the bug**
In v2, creating a stopwatch with `stopwatch.New()` (and no `stopwatch.WithInterval` argument) creates a stopwatch with an interval of 0, which causes some problems as the elapsed time never increases.

**Source Code**
https://github.com/charmbracelet/bubbles/blob/84a82dfeeed8089e0b9fd23a6344930b4f75eaf7/stopwatch/stopwatch.go#L65-L75

**Expected behavior**
According to the documentation, the interval should default to 1 second.