Replace @comment blocks with HTML comments (#50)

This reduces the places where content in the comment causes DocC to
silently mis-parse the comment and content after it.

The markdown parser for DocC first parses plain markdown syntax, and
then identifies places where the `@Comment { }` syntax delimits blocks.
That means it tries to parse the content of a comment as markdown first,
and then later it gets turned into a comment. The parsing of HTML style
comments happens as part of the first stage, which makes it less
fragile.

Using the DocC syntax for comments, a lot of the issues seem to come
from `}` appearing in commented-out in code listings, or from other
places where part of the comment is being interpreted as markdown and
preventing the later DocC parsing pass from treating that content as a
comment, or in some causes causing content after it to be missing. Using
the HTML comment syntax, the `-->` arrow used in comment-out formal
grammar conflicts with the end-of-comment marker, which I worked around
by changing them to a `->` arrow. We can probably delete most or all of
the commented-out formal grammar blocks, which seem to date back to the
original drafting of the book's grammar.

Some of the comment-parsing issues appear to be related to a known issue
in Swift-Markdown: swiftlang/swift-markdown#84

Author: amartini51

Sources/TSPL/TSPL.docc/GuidedTour/AboutSwift.md

@@ -51,12 +51,12 @@ Our goals for Swift are ambitious.
 We can’t wait to see what you create with it.
 
 
-@Comment {
+<!--
 This source file is part of the Swift.org open source project
 
 Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
 Licensed under Apache License v2.0 with Runtime Library Exception
 
 See https://swift.org/LICENSE.txt for license information
 See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-}
+-->

Sources/TSPL/TSPL.docc/GuidedTour/Compatibility.md

@@ -9,7 +9,7 @@ the default version of Swift that's included in Xcode 14.
 You can use Xcode 14 to build targets
 that are written in either Swift 5.7, Swift 4.2, or Swift 4.
 
-@Comment {
+<!--
   - test: `swift-version`
   
   ```swifttest
@@ -22,7 +22,7 @@ that are written in either Swift 5.7, Swift 4.2, or Swift 4.
   >> #endif
   << Just right
   ```
-}
+-->
 
 When you use Xcode 14 to build Swift 4 and Swift 4.2 code,
 most Swift 5.7 functionality is available.
@@ -52,12 +52,12 @@ you can migrate your code from Swift 4 to Swift 5.7
 one framework at a time.
 
 
-@Comment {
+<!--
 This source file is part of the Swift.org open source project
 
 Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
 Licensed under Apache License v2.0 with Runtime Library Exception
 
 See https://swift.org/LICENSE.txt for license information
 See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-}
+-->