Fix for assigning strings inside loop

Fixes #371

So this one was interesting. It's all to do with how strings get assigned: in the sample with the loop, the literal string "hello" only appears once in the source code (because it gets reused on each loop), so the parser only allocates a single value for that string. Which is fine.

BUT... assigning `hi` didn't actually create a new instance of the string, it just pointed the variable `hi` at that value, and the `rock` operator used this way actually modifies its argument instead of returning a copy. So `rock hi with "!"` is actually modifying the value that was allocated by the parser - and so on the next loop, `hi` already has the exclamation point from the previous loop.

Fixed by modifying the way the `Assign` statement works so that if you assign a string, it actually creates a copy of the string and assigns the variable to the copy, so the value allocated by the parser never gets modified.

Starship/Rockstar.Engine/RockstarEnvironment.cs

@@ -371,6 +371,7 @@ public Result Assign(Assign assign)
 
 	public Result Assign(Variable variable, Value value, Scope scope = Scope.Global) => value switch {
 		Functiön function => SetVariable(variable, MakeLambda(function, variable), Scope.Local),
+		Strïng strïng => SetVariable(variable, strïng.Clone(), scope),
 		_ => SetVariable(variable, value, scope)
 	};
 

Starship/Rockstar.Test/Rockstar.Test.csproj

@@ -1191,6 +1191,9 @@
     <None Update="programs\fixtures\conditionals\if-statement.rock">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </None>
+    <None Update="programs\fixtures\loops\assignment-inside-loop.rock">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 
   <ItemGroup>

Starship/Rockstar.Test/programs/fixtures/loops/assignment-inside-loop.rock

@@ -0,0 +1,11 @@
+(writes: Hello!Hello!Hello!)
+for counter in 3
+hi = "Hello"
+rock hi with "!"
+write hi
+
+for counter in 3
+hi = 1
+hi is with 5
+write hi
+(writes: 666)