Add opt-in feature of enforcing properly constructed Given-When-Then scenarios

[Suremaker]

Description

As a follow up to #167, the LightBDD should offer and opt-in feature enforcing developers to write properly constructed scenarios, where:

• each scenario will start with 0 or more SETUP steps,
• followed by 1 or more GIVEN steps,
• followed by 1 or more WHEN steps (configurable to exactly 1?),
• followed by 1 or more THEN steps.

When feature is enabled, any scenario written in any style (basic/extended/compact way) should fail if step types are in wrong order or missing.

The composite steps behavior should be rethink yet, but there are multiple possible implementations:

1. composite sub-steps should be verified independently from parent step, i.e

GIVEN I purchased a TV
    GIVEN I have a basket
    GIVEN I put TV to the basket
    WHEN I go to the checkout
    WHEN I pay
    THEN The purchase should be made
WHEN I request an invoice for the purchase
THEN I should receive the invoice

2. composite sub-steps should start with the same type as the composite itself

GIVEN a basket with a TV
    GIVEN I have a basket
    GIVEN I put TV to the basket
WHEN I finalize the transaction
    WHEN I go to the checkout
    WHEN I pay
    THEN The purchase should be made
WHEN I request an invoice for the purchase
THEN I should receive the invoice

2. composite sub-steps should be only of the same type as the composite itself

GIVEN a basket with a TV
    GIVEN I have a basket
    GIVEN I put TV to the basket
WHEN I finalize the transaction
    WHEN I go to the checkout
    WHEN I pay
WHEN I request an invoice for the purchase
THEN I should receive the invoice

Progress

• Feature is implemented,
• Ensured backward-compatibility,
• Ensured good debugging experience