Add support for ASCII Art SVG (aasvg) diagrams

[eckelmeckel]

IETF's Kramdown supports ASCII Art SVG (aasvg) diagrams/images. It would be nice if TCG's pandoc engine has support for it, too. An example can be found here: https://github.com/ietf-scitt/draft-birkholz-scitt-architecture/blob/main/draft-birkholz-scitt-architecture.md?plain=1#L237:

                    .----------.
                   |  Artifact  |
                    '----+-----'
                         v
                    .----+----.  .----------.
Issuer       -->   | Statement ||  Envelope  |
                    '----+----'  '-----+----'
                         |             |       +------------------+
                          '----. .----'        | DID Key Manifest |
                                |              | (decentralized)  |
                                v              +---+----------+---+
                             .--+--.   Sign Claim  |          |
                            | Claim +<------------'           |
                             '--+--'                          |
                                |            +--------------+ |
                             .-' '---------->+ Transparency | |
                            |   .-------.    |              | |
Transparency -->            |  | Receipt |<--+   Registry   | |
     Service                |   '---+---'    +-------+------+ |
                             '-. .-'                 |        |
                                |                    |        |
                                v                    |        |
                          .-+-------+-.              |        |
                         | Transparent |             |        |
                         |    Claim    |             |        |
                          '-----+-----'              |        |
                                |                    |        |
                                |'------.     .------)-------'
                                |        |   |       |
                                |        v   v       |
                                |   .----+---+-----. |
Verifier      -->               |  / Verify Claim /  |
                                | '--------------'   |
                                v                    v
                       .--------+---------.  .-------+---------.
Auditor       -->     / Collect Receipts /  / Replay Registry /
                     '------------------'  '-----------------'

[chrisfenner]

I grabbed the PDF output in the CI following this PR, and it looks like it didn't render as expected:

https://github.com/TrustedComputingGroup/pandoc/actions/runs/13798955347