add PolySharp for compiler features

Author: Elektryk-Andrzej

Code/Helpers/Compiler/CompilerRequiredAttributes.cs

@@ -13,7 +13,7 @@ public static class StringExtensions
     [Pure]
     public static string LowerFirst(this string str)
     {
-        return str.Substring(0, 1).ToLower() + str.Substring(1);
+        return str[0].ToString().ToLower() + str[1..];
     }
 
     // python ahh

Code/Helpers/Compiler/IsExternalInit.cs.cs

@@ -18,12 +18,9 @@ public class HTTPGetMethod : YieldingReferenceReturningMethod<JObject>, ICanErro
     public override string Description =>
         "Sends a GET request to a provided URL and returns the response as a JSON object.";
 
-    public string[] ErrorReasons =>
-    [
-        "Fetched value from the URL is not a valid JSON object.",
-        nameof(UnityWebRequest.Result.ConnectionError),
-        nameof(UnityWebRequest.Result.DataProcessingError),
-        nameof(UnityWebRequest.Result.ProtocolError)
+    public string[] ErrorReasons => [
+        ..HTTPPostMethod.HttpErrorReasons, 
+        "Provided response was not a valid JSON object."
     ];
 
     public override Argument[] ExpectedArguments { get; } =

Code/Helpers/Compiler/NotNullWhenAttribute.cs

@@ -92,6 +92,10 @@
         <PackageReference Include="NCalc" Version="1.3.8" />
         <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
         <PackageReference Include="Northwood.LabAPI" Version="1.1.5" />
+        <PackageReference Include="PolySharp" Version="1.15.0">
+          <PrivateAssets>all</PrivateAssets>
+          <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
     </ItemGroup>
     <ItemGroup>
       <Folder Include="Code\" />