Discussion + Feature Request: Implement Intellisense for DFDL Functions

# 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.

Read the DFDL function documentation at the two linked sections and inspect the extension's existing IntelliSense implementation. Define the function-suggestion scope, add concrete acceptance criteria and subtasks, then verify that supported DFDL functions appear correctly in IntelliSense.

Written by the indexing model from the issue text.