AK: Add String{,Utils}::to_snakecase()

This is an improved version of WrapperGenerator's snake_name(), which
seems like the kind of thing that could be useful elsewhere but would
end up getting duplicated - so let's add this to AK::String instead,
like to_{lowercase,uppercase}().

AK/String.h

@@ -131,6 +131,7 @@ public:
 
     String to_lowercase() const;
     String to_uppercase() const;
+    String to_snakecase() const;
 
     bool is_whitespace() const { return StringUtils::is_whitespace(*this); }