Add environment variable support for MAX_LENGTH initialization

[balazser]

Hello,

Could we add support for initializing the MAX_LENGTH value using an environment variable in the Better Exceptions repository? 🙏

This would allow users to customize the MAX_LENGTH value according to their specific requirements without touching the code.

Currently, the MAX_LENGTH value is defined in the Python code. This makes it difficult for users to customize the value according to their needs. By adding support for environment variables, users could set the MAX_LENGTH value by using an environment variable such as BE_MAX_LENGTH.