Support SSML

[grhoten]

Is your feature request related to a problem? Please describe.
This request is similar to #15. While message formatting should not have to natively support SSML, it should not prevent such support.

Describe the solution you'd like
I'd like native support for it.

Describe why your solution should shape the standard
Message formatting should be localizable, and the ability to mark it up to support a spoken context in addition to a printable context is important. The printable form would always omit the SSML markup, and only the spoken form would be aware of SSML. This support will also help people with disabilities that are unable to view the printable form while getting a better experience listening to a given message. Though there should be guidance on how the printable form strips the SSML markup. From my experience, we just collapse the spaces to 1, but there may be other opinions.

Additional context or examples
Here are examples of things supporting SSML
Amazon Alexa
Google Assistant
Microsoft Cortana

[aphillips]

Marking as resolved candidate because we addressed this (and other markup syntaxes) with open/close functions. Note that open/close remains a hot topic.

There should probably be some documentation about markup and other syntaxes, perhaps in a user guide.

[aphillips]

Closing resolve-candidates per discussion in 2023-07-24 call