feat: Adding Strict Package

[yhakbar]

Description

Adds strict package.

This allows users to opt-in for deprecations as breaking changes in Terragrunt.

Users can either use the global TG_STRICT_MODE to make Terragrunt throw an error when it encounters a deprecation, or use the relevant TG_STRICT_ environment variable to opt-in for a specific deprecation.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added strict package.

Migration Guide

If users would like to make Terragrunt throw an error when it encounters a deprecation, they can set the TG_STRICT_MODE environment variable to true.
If they would like to opt-in for a specific deprecation, they can set the relevant TG_STRICT_ environment variable to true.

Neither are required and users can continue to use Terragrunt as they have been.