feat(WidgetMember): update display_name method to include global_name

[Kevin-OVI]

Summary

This is a follow-up to #1123 . I had suggested to also apply that change to WidgetMember as it also has a display_name property, but that PR got merged without it.
This PR applies that same change as Member to WidgetMember : the display_name property will return the name using the following hierarchy:

1. Guild specific nickname
2. Global Name (also known as 'Display Name' in the Discord UI)
3. Unique username

This is a Code Change

• I have tested my changes.
• I have updated the documentation to reflect the changes.
• I have run task pyright and fixed the relevant issues.

[Skelmis]

I feel like 2 & 3 should maybe be swapped at first look, but this is a choice for others to agree or disagree with. Consider this comment an approval if others like the current order.

[Kevin-OVI]

I don't think so, this is the order used by User and Member in the original PR, and also the one used in the Discord UI.

[EmreTech]

Yeah, looking back at it, this was actually needed in the original PR. Sorry about that!