Create universal parser

[istride]

Introduces a "universal" parser that is intended to convert between spreadsheets and JSON and be useful as a basis for any application-specific purpose (e.g. ParentText/ChatbotBuilder, AppBuilder).

A concerted effort has been made to preserve the existing conversion behaviour of the current ParentText-specific parser, for backwards-compatibility. A major exception has been made for templates, which are no longer interpreted during the conversion, just copied verbatim. An overview of the conversion behaviour can be found in docs/notation.md.

The universal parser does not use Pydantic models during conversion. Instead, the parser opportunistically converts strings to the JSON types. For example, if a string can be converted to a number it will be; if not the parser will fall back to different conversion methods until one works, or none of them work and the string is just passed through. A method for directing the conversion process is considered out of scope.

Metadata is stored within the JSON, in a top-level property with key _idems, to direct the conversion back to sheets. Currently, only the original table headers are stored as metadata, to preserve their order and layout.

Two CLI sub-commands have been added:

• sheets-to-uni: converts a spreadsheet to nested ("universal") JSON
• uni-to-sheets: reverse operation back to spreadsheet; currently limited to ODS output

Examples:

rpft sheets-to-uni 1hcH8pFdiHZN0UvZgyv3Zht9ARBTx-VXhNBI2o8L7fHU T_content.json
rpft uni-to-sheets T_content.json T_content.ods

Supporting other output formats is considered out of scope for this PR.