Add charmap for "+" to "plus"

[najmul-islam]

• add "+" to "plus".
• in mobile site mostly use "+" sign like "Samsung Galaxy S22+".
• for this sign "+" use for "plus" keyword.
• in my mobile-description project need to "+" sing to "plus" keyword but in slugify it absence. so I added it for my and other project like this.

[NaveedMaq]

• How often have you seen this usecase in your experience?

[najmul-islam]

I have seen this many websites. like gsmarena , mobiledokan