fix(fields): Make SKIP_SETUP a Symbol; remove Sentinel class (#6919)

We introduced the SKIP_SETUP sentinel value when converting
Field and its subclasses to ES6 class syntax, because super
must be called before any other code in a subclass
constructor, breaking the previous mechanism where subclasses
would set some properties before calling their superclass
constructor.

SKIP_SETUP was a singleton value of class Sentinel.

Recently, in PR #6639 @btw17 introduced the isSentinel type
predicate to improve the typing of Field.  Unfortunately, there
were some aspects of this change that were not very elegant:

- isSentinel was declared as a static method on Field (rather
  than on Sentinel itself).
- It only checks against the specific value SKIP_SETUP,
  rather than checking if the argument is instanceof Sentinel
  (though perhaps this is for efficiency?)

Additionally - as a result of the migration from ES6 to TS, and
predating PR #6639 - The signature for the Field constructor's
first argument was typed T|Sentinel, with subclass constructors
generally being <some type(s)>|Sentinel.

This creates a small problem when attempting to port Fields from
core to plugins, because Sentinel is not reexported by
core/utils.ts (and therefore not from core/blockly.ts either).

The latter problem could be solved simply by reexporting Sentinel,
or by having plugin constructors not accept SKIP_SETUP (though
this potentially makes them more difficult to subclass), but
neither is particularly desirable.

Instead, this PR proposes that we:

- Make Field.SKIP_SETUP a (unique) Symbol.
- Change the value argument to the Field constructor to accept
  T|typeof Field.SKIP_SETUP - where typeof Field.SKIP_SETUP is
  (like a literal type) a type that accepts just the single
  value SKIP_SETUP.
- Remove the Sentinel class and core/utils/sentinel.ts.

Not treating this as a breaking change:

- Removes Field.isSentinel - though this addition has not yet
  been published, so it can only break our own as-yet-unreleased
  code in samples.

- Changes the type of Field.SKIP_SETUP and the first argument
  of the Field constructor from Sentinel to typeof SKIP_SETUP
  (a unique Symbol) - but given that Sentinel has never been
  exported this should not break any actual external code.

core/utils/sentinel.ts

@@ -1,25 +0,0 @@
-/**
- * @license
- * Copyright 2022 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-/**
- * A type used to create flag values.
- *
- * @class
- */
-import * as goog from '../../closure/goog/goog.js';
-goog.declareModuleId('Blockly.utils.Sentinel');
-
-
-/**
- * A type used to create flag values.
- */
-export class Sentinel {
-  /**
-   * Provide a unique key so that type guarding properly excludes values like
-   * string.
-   */
-  UNIQUE_KEY?: never;
-}