feat: Add `-y` / `--yes` flag for non-interactive CI/CD execution

[bolespolo4]

🧩 Feature Request

Problem

flutter_flavorizr currently requires interactive user input during execution.
This makes it impossible to run in automated pipelines such as GitHub Actions,
Bitrise, or any CI/CD environment where stdin is not available.

Proposed Solution

Add a -y / --yes CLI flag that bypasses all interactive prompts by
assuming default/affirmative answers, enabling fully non-interactive execution.

Example usage:
flutter pub run flutter_flavorizr -y

Motivation

This is a common pattern in CLI tools (e.g. apt-get install -y, npm init -y)
and is expected behavior for any tool meant to be used in automated workflows.

Many teams want to integrate flutter_flavorizr into their CI/CD pipelines
but are currently blocked by the interactive prompt requirement.

Proposed Behavior

• When -y is passed: all prompts are bypassed using safe default values
• When -y is absent: existing behavior is completely preserved (no breaking change)
• Exit with a non-zero code on failure so pipelines can detect errors reliably

Existing Flag Note

I noticed the -f / --force flag exists — I want to align with you on
whether -y should be additive or an alias/extension of -f before
implementing anything.

Additional Context

I am willing to implement this feature and open a PR following the
project's contribution guidelines.