Feature Request: Implement Intellisense for DFDL Functions

Problem Description

The extension currently doesn't provide IntelliSense suggestions for functions. See https://daffodil.apache.org/docs/dfdl/#_Toc62570225 and https://daffodil.apache.org/docs/dfdl/#_Toc62570222.

A feature that can be added to IntelliSense functionality is implementing functions.

Proposed Solution

Proposed solution will be explored further via discussions in this ticket. Additional subtasks will be added.

Acceptance Criteria

Not applicable just yet. Most likely, a checklist will be added.

Impact and Benefits

This feature will expand out the extension's IntelliSense capabilities and implement more DFDL functionality.

Open Questions / Discussion Points

Please discuss anything relating to the additions in this ticket.