Allow for synchrous DAC output

Re this thread:
https://forums.adafruit.com/viewtopic.php?t=198312

Currently each channel is updated when it is set:
```python
mcp4728.channel_a.value = 100
mcp4728.channel_b.value = 75
mcp4728.channel_c.value = 50
mcp4728.channel_d.value = 25
```
via this line:
https://github.com/adafruit/Adafruit_CircuitPython_MCP4728/blob/d907e3e0baa15cd50cc647d80261dac77aeb68fe/adafruit_mcp4728.py#L347

Add capability and example to allow multiple channel values to be updated synchronously.

Maybe something similar to `auto_write` in NeoPixel/Dotstar usage?

Read the linked forum thread and inspect adafruit_mcp4728.py around line 347 to understand the current per-channel write behavior. Define the synchronous update capability and add an example showing multiple channel values being committed together; it is done when all requested channels update simultaneously without individual writes taking effect first.

Written by the indexing model from the issue text.