Add uppercase character check to SlashCommandBuilder and ApplicationCommandOptionProperties (#339)

src/Discord.Net.Core/Entities/Interactions/ApplicationCommandOption.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text.RegularExpressions;
 
 namespace Discord
@@ -29,6 +30,9 @@ namespace Discord
                 if (!Regex.IsMatch(value, @"^[\w-]{1,32}$"))
                     throw new FormatException($"{nameof(value)} must match the regex ^[\\w-]{{1,32}}$");
 
+                if (value.Any(x => char.IsUpper(x)))
+                    throw new FormatException("Name cannot contain any uppercase characters.");
+
                 _name = value;
             }
         }

src/Discord.Net.Core/Entities/Interactions/SlashCommands/SlashCommandBuilder.cs

@@ -40,6 +40,9 @@ namespace Discord
                 if (!Regex.IsMatch(value, @"^[\w-]{1,32}$"))
                     throw new ArgumentException("Command name cannot contain any special characters or whitespaces!", nameof(value));
 
+                if (value.Any(x => char.IsUpper(x)))
+                    throw new FormatException("Name cannot contain any uppercase characters.");
+
                 _name = value;
             }
         }