Wait screen settled

**Is your feature request related to a problem? Please describe.**

Making tapes for LLM TUIs is not easy with the current set of commands, I would like to have a `WaitScreenSettled ` that waits the for the screen to remain unchanged for some amount of time before resuming the execution of the tape.

I have a first implementation of this here: https://github.com/rumpl/vhs/tree/screen-settled but wanted to open an issue before, see what y'all think.

**Describe the solution you'd like**

I should be able to do something like this:

```tape
Type crush
Enter
WaitScreenSettled 1s

Type "Hello"
Enter
WaitScreenSettled 1s

Sleep 1s
```

**Describe alternatives you've considered**

I've tried waiting for some text to appear but LLMs be LLMs and their output is random

Review the first implementation on the screen-settled branch linked in the issue, then trace how tape commands are parsed and executed. Verify that WaitScreenSettled waits for the terminal screen to remain unchanged for the requested duration before continuing, using the example tape sequence as the completion criterion.

Written by the indexing model from the issue text.