Add tests for ThemeJSONParser

[roseg43]

Additional context Write tests to confirm

• toCssCustomPropertyString()
  • Correctly parses kebab-case, camelCase, TitleCase
  • Correctly parses
    • nested properties
    • non-nested properties
    • settings.custom properties
    • settings.typography.fontFamilies properties
    • settings.typography.fontSizes properties
    • settings.spacing.spacingSizes properties
    • settings.color.palette settings.color.gradients properties
  • toArray() generates a flat array of data objects with label, description props.
• Extension does not break if invalid or falsy themeJson is supplied on object instantiation.

[roseg43]

Tests for

• Properly handles when a property group hasn't been defined (nested, non-nested)

Have been added in #19