Fix gutter icons fields (#4549)

Fixes #4540

### Changes

Do not create verification trees for non-constant fields that can never
trigger verification

### Testing

Added an XUnit test

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyLanguageServer.Test/GutterStatus/SimpleLinearVerificationGutterStatusTester.cs

@@ -14,6 +14,24 @@ public class SimpleLinearVerificationGutterStatusTester : LinearVerificationGutt
   // the test will fail and give the correct output that can be use for the test
   // Add '//Replace<n>:' to edit a line multiple times
 
+  [Fact]
+  public async Task Fields() {
+    var markedSource = @"
+| :method Test() {
+| :  assert true;
+| :}
+| :
+| :class A {
+| :  ghost var B: seq<int>
+| :  var C: array<int>
+| :
+| :  method Test() {
+| :    assert true;
+| :  }
+| :}";
+    await VerifyTrace(markedSource, false, intermediates: false);
+  }
+
   [Fact]
   public async Task GitIssue4287GutterHighlightingBroken() {
     await VerifyTrace(@"

Source/DafnyLanguageServer/Workspace/VerificationProgressReporter.cs

@@ -95,7 +95,7 @@ void AddAndPossiblyMigrateVerificationTree(VerificationTree verificationTree) {
               continue;
             }
 
-            if (member is Field) {
+            if (member is ConstantField) {
               var constantHasNoBody = member.RangeToken.EndToken.line == 0;
               if (constantHasNoBody) {
                 continue; // Nothing to verify