Add SVG format to the spec.

[snianu]

Closes #92

For normative changes, the following tasks have been completed:

• [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

• [X] WebKit (https://lists.webkit.org/pipermail/webkit-dev/2021-August/031940.html)
• [X] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=1110511)
• [X] Gecko (https://github.com/mozilla/standards-positions/issues/549)

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[snianu]

@EdgarChen Gentle ping..

[annevk]

This doesn't seem to define any of the sanitization required...