support ANSI color output of `pkl eval`

I was watching Viktor's [review of pkl](https://www.youtube.com/watch?v=Nm1ioWPRRVQ) and the lack of syntax highlighting shouted me when running `pkl eval` on the command line.

I'm so used to color based syntax highlighting in all languages it really shouts to me when there's no color

I wonder if we should have an optional colour option to `pkl eval` that could use ANSI colours to make the output look nicer?

Start at the `pkl eval` command-line entry point and inspect how its output is currently formatted. Determine the scope of an optional ANSI color mode and how syntax highlighting should appear, then verify the resulting CLI output with representative evaluations.

Written by the indexing model from the issue text.