Add unit test project and some early tests

PluralKit.Bot/Proxy/ProxyTagParser.cs

@@ -8,10 +8,13 @@ namespace PluralKit.Bot
 {
     public class ProxyTagParser
     {
-        public bool TryMatch(IEnumerable<ProxyMember> members, string input, out ProxyMatch result)
+        public bool TryMatch(IEnumerable<ProxyMember> members, string? input, out ProxyMatch result)
         {
             result = default;
             
+            // Null input is valid and is equivalent to empty string
+            if (input == null) return false;
+            
             // If the message starts with a @mention, and then proceeds to have proxy tags,
             // extract the mention and place it inside the inner message
             // eg. @Ske [text] => [@Ske text]