Use StringBuilder.Append(Char) instead of coverting char to string

[alaatm]

No description provided.

[EgorBo]

it'd be nice to for static analyzer to find patterns like stringBuilder.Append(charVar.ToString()) and suggest to use the char overload instead

[stephentoub]

t'd be nice to for static analyzer to find patterns like stringBuilder.Append(charVar.ToString()) and suggest to use the char overload instead

Yup, and not just for char, but in general Append(primitive.ToString()) should just be Append(primitive). It's already on the list at #30740 😄 If you have other ideas, please do comment on that issue to suggest them.
cc: @terrajobst