[V3] Directory Loading and Begin Implementing Lazy-Loading

Problem:

As it stands, Dasel does not lazy load to any extent, which can cause performance issues for large datasets.

Solution (Getter-Setters):

Dasel already has groundwork implemented for dynamically-loading properties (implemented for orderedmaps) in model.Value:

• https://github.com/TomWright/dasel/blob/5ac2dab3c95476c27de0026f5ea2a12be384523e/model/value_map.go#L85-L127
• https://github.com/TomWright/dasel/blob/5ac2dab3c95476c27de0026f5ea2a12be384523e/model/value_map.go#L41-L60

Using this existing getter-setter structure, you could add support for lazily-loaded types that only load into memory on demand.

Use Case: Directory Loading

A simplistic first use-case would be directory loading. When parsing the directory, you first only load the directory tree into memory as a map. Then when leaves (files) are accessed, you load those on demand. This would give you a poor man's lazy-loading method for large datasets by allowing end users to segment their files.