Prevent creating variables with keywords as names.

From MeteorDevelopment/starscript#27

Author: GreemDev

src/Lib/Public/Values/ValueMap.cs

@@ -1,10 +1,13 @@
 using System.Collections;
+using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
 
 namespace Starscript;
 
 public class ValueMap : IReadOnlyDictionary<string, Func<Value>>
 {
+    public static readonly ImmutableArray<string> Keywords = ["null", "true", "false", "and", "or"];
+    
     private readonly Dictionary<string, Func<Value>> _entries;
 
     /// <summary>
@@ -22,6 +25,12 @@ public ValueMap(Dictionary<string, Func<Value>>? entries = null)
     /// </summary>
     public ValueMap SetRaw(string name, Func<Value> value)
     {
+        if (Keywords.Contains(name))
+            throw new StarscriptException($"The name of a variable cannot be a keyword. " +
+                                          $"Disallowed words: [{
+                                              string.Join(", ", Keywords.Select(x => $"\"{x}\""))
+                                          }]");
+        
         _entries[name] = value;
         return this;
     }

src/TestProgram/Program.cs

@@ -60,6 +60,17 @@ internal static void Main()
             // StarscriptHypervisor should error here, as the source string calls a function that is defined as a local, so it is cleared when the script execution ends.
             // NotImplementedException is not caught as that is an error case, that being StarscriptHypervisor *not* erroring.  
         }
+        
+        try
+        {
+            hypervisor.Set("true", Value.Null);
+
+            throw new NotImplementedException();
+        }
+        catch (StarscriptException)
+        {
+            Log("Successfully errored when trying to set a variable name to a keyword.");
+        }
 
         script.Dispose();
 #endif