Add a utility tool for previewing event symbol rendering

[stmontgomery]

This adds a development-only utility executable target to the package which allows previewing the event symbols rendered using various console output styles.

Motivation:

This is a tool I made and found useful while working on #983.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test