✨ rework confidence scores to be easily comparable

[ianardee]

Description

Rework confidence scores, they are now always a class rather than a string.

Added >= and <= operators to compare confidence scores.

Removed the low?, medium?, etc methods as we want users to harmonize all comparison operators.

Fixes #204

Usage:

my_field = response.inference.result.fields.get('my_field')
my_field.confidence == Mindee::Parsing::V2::Field::FieldConfidence::MEDIUM
my_field.confidence >= Mindee::Parsing::V2::Field::FieldConfidence::LOW
my_field.confidence <= Mindee::Parsing::V2::Field::FieldConfidence::HIGH

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Requires a change to the official Guide documentation.