Having a general/default limit_memory setting

## Feature Request

I would like to prevent people adding tests that take up too much memory.

Thus I would like to set a global `limit_memory` setting, that would be overwritten locally by the decorator.

It may not enabled by default but only when being executed with `--memray` (or toggled on/off via a new arg).

Cheers and thanks for the great plugin!

Start by locating the existing limit_memory decorator and the --memray entry point. Trace how settings are read and applied, then determine how a global limit should interact with a local decorator override and whether activation is tied to --memray or a new argument. Done means the behavior is covered by the plugin's existing tests.

Written by the indexing model from the issue text.