New "must call base" analyzer. (#5266)

* New "must call base" analyzer.

This enforces that you actually call base when overriding stuff. This is intended for base methods like entity system's, where server/client systems overriding shared ones SHOULD call Initialize() and such.

* Add MustCallBase to entity system methods

Robust.Analyzers.Tests/MustCallBaseAnalyzerTest.cs

@@ -0,0 +1,92 @@
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
+using Microsoft.CodeAnalysis.Testing.Verifiers;
+using NUnit.Framework;
+using VerifyCS =
+    Microsoft.CodeAnalysis.CSharp.Testing.NUnit.AnalyzerVerifier<Robust.Analyzers.MustCallBaseAnalyzer>;
+
+namespace Robust.Analyzers.Tests;
+
+[Parallelizable(ParallelScope.All | ParallelScope.Fixtures)]
+[TestFixture]
+public sealed class MustCallBaseAnalyzerTest
+{
+    private static Task Verifier(string code, params DiagnosticResult[] expected)
+    {
+        var test = new CSharpAnalyzerTest<MustCallBaseAnalyzer, NUnitVerifier>()
+        {
+            TestState =
+            {
+                Sources = { code }
+            },
+        };
+
+        TestHelper.AddEmbeddedSources(
+            test.TestState,
+            "Robust.Shared.IoC.MustCallBaseAttribute.cs"
+        );
+
+        // ExpectedDiagnostics cannot be set, so we need to AddRange here...
+        test.TestState.ExpectedDiagnostics.AddRange(expected);
+
+        return test.RunAsync();
+    }
+
+    [Test]
+    public async Task Test()
+    {
+        const string code = """
+            using Robust.Shared.Analyzers;
+
+            public class Foo
+            {
+                [MustCallBase]
+                public virtual void Function()
+                {
+
+                }
+
+                [MustCallBase(true)]
+                public virtual void Function2()
+                {
+
+                }
+            }
+
+            public class Bar : Foo
+            {
+                public override void Function()
+                {
+
+                }
+
+                public override void Function2()
+                {
+
+                }
+            }
+
+            public class Baz : Foo
+            {
+                public override void Function()
+                {
+                    base.Function();
+                }
+            }
+
+            public class Bal : Bar
+            {
+                public override void Function2()
+                {
+                }
+            }
+            """;
+
+        await Verifier(code,
+            // /0/Test0.cs(20,26): warning RA0028: Overriders of this function must always call the base function
+            VerifyCS.Diagnostic().WithSpan(20, 26, 20, 34),
+            // /0/Test0.cs(41,26): warning RA0028: Overriders of this function must always call the base function
+            VerifyCS.Diagnostic().WithSpan(41, 26, 41, 35));
+    }
+}