Add mise activate command to user's shell config

Overview

At the end of the mise migration, users need to manually add the mise activate command to their shell config files, but some users miss this step and this causes confusion when mise isn't available. We should consider adding this to their shell config as part of the migration. Previously we didn't opt for this since some users won't like their shell config being updated by GDK.

Impacted categories

The following categories relate to this issue:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.