Correct image placeholder text

[cwickham]

When "dropping" an image into the VS Code Editor with the Quarto extension the following markdown is generated:

![Alt text](path/to/image.png)

It would be more correct for Alt text to say Caption, i.e.

![Caption](path/to/image.png)

It might also be worth considering whether the placeholder should include alt text, e.g. perhaps you should get:

![Caption](path/to/image.png){fig-alt="Alt text"}