Also add DarkMauve and DarkFlagshipTerrastruct as explicit theme options

DarkMauve was already in the theme list

Also:

- Adds is_nonempty_string helper function
- Appends random number for diagram file name
- Use d2 file extension not txt for input
- Allow alternate syntax for attribute classes on code blocks

Also:

- Add support for reading d2 diagrams from external files using `file` parameter. Block text is ignored if file parameter is supplied.
- Add support for alternate code block syntax without curly braces.
- Insert rendered diagrams into the Pandoc mediabag when `embed_type="link"`
- Refactor to add helper functions `setPreD2RenderOptions` and `setD2RenderFormat`

Also

- add animate_interval parameter
- drop setD2RenderFormat helper function
- use quarto.project.output_directory to set default folder
- revise check for d2 code block class
- replace codeblock text if file is provided
- add support for tala layout

Also:

- add check to see if file exists
- Improve handling of code block formatting (when echo=true)
- remove duplicative processing for options.pad and options.animate_interval

Based on https://github.com/shafayetShafee/add-code-files

Also add external d2 file to test file argument