Transform between TemplateMark and Slate DOM

[jeromesimeon]

Is your feature request related to a problem? Please describe. The ability to create a Slate DOM from a template grammar rather than a contract (i.e., from TemplateMark rather than CiceroMark). The ability to transform the corresponding Slate DOM back to template mark (roundtrip).

Describe the solution you'd like

• A pair of transforms parallel to the CiceroMark <-> Slate DOM one but for TemplateMark
• Should share code for the common part of the DOM (i.e., for CommonMark)
• Should roundtrip

Additional context

• This would be a building block toward design mode, assuming this is still the plan for template grammar editing, allowing to use a rich text editor for the template rather than the contract. See: https://github.com/accordproject/web-components/issues/64