Wrapped Variants of Conditional blocks

[jeromesimeon]

Is your feature request related to a problem? Please describe.

Provide new CiceroMark nodes for variables corresponding to conditional blocks in templates.

CiceroMark variables can be either seen as plain text or wrapped (e.g., <variable id="rate" value="3.5"/>).

Conditional blocks in CiceroMark can also be wrapped, but cannot be easily distinguished from regular variables.

Conditional variables corresponding to conditional blocks are not reflected in the Slate AST, which means they cannot easily be edited by the UI components.

Describe the solution you'd like

• A new kind of AST node in CiceroMark: ConditionalVariable
• A new kind of AST node in Slate: conditional
• The ability to parse/draft them. Suggested formats would be similar to variables but conditionals <if id="varName" value=""/>
• Translations to/from markdown, Cicero and Slate.

Additional context
Support for editing of conditional and list variables in Cicero UI:
accordproject/cicero-ui#233