Allow certain directives

[emilyychenn]

Bug/issue #, if applicable: #591

Summary

Allow some directives in doc comments. This feature is motivated by allowing the @Snippet directive in doc comments, as was noted here: https://forums.swift.org/t/using-snippets-in-documentation-comments-not-possible/65046.

Testing

With a documentation catalog

In a documentation catalog with symbol graph files and documentation extension files:

1. In the documentation extension file for the module symbol: add a few different directives in the comments. For example:

   /// @TabNavigator {
   ///     @Tab("Tab 1") {
   ///         @Row {
   ///             @Column {
   ///                 Hello!
   ///             }
   ///
   ///             @Column {
   ///                 Hello there!
   ///             }
   ///         }
   ///
   ///         Hello there.
   ///     }
   ///
   ///     @Tab("Tab 2") {
   ///         Hey there.
   ///
   ///         @Small {
   ///             Hey but small.
   ///         }
   ///     }
   /// }
   

2. Preview the documentation for that docc catalog with docc preview /path/to/Example.docc
3. Navigate to that page in the preview. The added directives should render as expected, and there should be no warnings about unsupported directives.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[emilyychenn]

@swift-ci please test

[ethan-kusters]

@swift-ci please test

[emilyychenn]

@swift-ci please test

[ethan-kusters]

Looks great! Thank you!

[ethan-kusters]

@swift-ci please test