feature: fallback value for the env shortcode

[cscheid]

Discussed in #8315

^{Originally posted by aronatkins January 17, 2024}

Description

The env shortcode does not allow a fallback value.

In R, we might:

Sys.getenv("FRUIT", "apples")

In Python, we might:

os.environ.get("FRUIT", "apples")

In simple Quarto documents (using the Markdown engine), we do not have either of these options. If the document were accompanied by a _quarto.yml file (and seen as a "project"), that document could use an _environment file to provide default environment values. The _environment file is only processed for projects, not standalone documents.

Proposal: Extend the env shortcode to allow a fallback value.

(awesome website)[{{< env TARGET_URL "https://quarto.org/" >}}]