[front end] Change Template class to extend Code.

This is part of a series of CLs that will standardize CFE error
reporting to always use `codeFoo.withArguments(...)` when reporting
errors that take arguments and `codeFoo` when reporting errors that
don't take arguments, rather than `templateFoo.withArguments(...)`
when reporting errors that take arguments and `messageFoo` when
reporting errors that don't take arguments. This change will have two
advantages:

- It will lend greater consistency to the CFE codebase, by allowing
  the same `code...` objects to be used both to name error codes
  (e.g., in test expectations) and to report errors. This will allow
  everything associated with a certain error code to be found using a
  single invocation of "Find References" in the editor, rather than
  having to search separately for uses of the code and the message or
  template.

- It should hopefully make the experience of writing code that reports
  errors more pleasant, since it will no longer be necessary to look
  up an error to see whether it takes arguments before using it;
  instead, the developer will be able to type the name of the message
  `code...` declaration, and then use autocompletion to see whether
  `.withArguments(...)` is required.

In this CL, the `Template` class is changed so that it extends
`Code`. In follow up CLs, this will allow the `Template` objects to
serve the dual role of being either a template or an error code, in
much the same way that for errors that don't take arguments, the
`MessageCode` objects serve the dual role of being either an error
code or an error message.

Change-Id: I2e25419e47d2b0b601488b929ab5415d7039abea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443002
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/codes.dart

@@ -104,8 +104,8 @@ class MessageCode extends Code implements Message {
   }
 }
 
-class Template<T> {
-  final String messageCode;
+class Template<T> extends Code {
+  String get messageCode => name;
 
   final String problemMessageTemplate;
 
@@ -114,13 +114,13 @@ class Template<T> {
   final T withArguments;
 
   const Template(
-    this.messageCode, {
+    super.name, {
     this.correctionMessageTemplate,
     required this.problemMessageTemplate,
     required this.withArguments,
-    int index = -1,
-    List<String>? analyzerCodes,
-    CfeSeverity severity = CfeSeverity.error,
+    super.index = -1,
+    super.analyzerCodes,
+    super.severity = CfeSeverity.error,
   });
 
   @override