Update swift-markdown from caafc56d to 97df6e2

Updates swift-markdown to include:

    commit 97df6e2812adcf8698204ca5f0756563ef36e5c1
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Sat May 21 14:02:22 2022 -0700

        Correctly set `indexInParent` in `Markup.child(at:)` (#45)

        This fixes a recent regression where `Markup.child(at:)` began returning
        markup with incorrect metadata resulting in out-of-bounds errors.

        The `indexInParent` value of a child is unrelated to the parent's
        `indexInParent`. This was missed initially because tests were only
        checking for children of the first item where `indexInParent` would
        be 0. A new test has been added that asserts `Markup.child(at:)` returns
        correct values for children of nested items as well.

    commit 7a7c59d1160fddd23e2379ec5ee8c71df7fd0b3b
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Fri May 20 18:53:24 2022 -0700

        Improve performance of `Markup.child(at:)` method (#44)

        Improves the performance of `Markup.child(at:)` by refactoring to removing
        the need to iterate over all previous elements in the child array.

    commit 36cf89e36e94b025dbe37d82fe2a7da3f39d4204
    Author: Franklin Schrans <fschrans@apple.com>
    Date:   Wed Mar 23 14:38:18 2022 +0100

        Remove extraneous print in test (#33)

    commit e50693584310a9190071c96c839485b6f2376832
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Mon Mar 21 10:42:02 2022 -0700

        Publish Swift Markdown's documentation to GitHub pages (#32)

        * Adopt the Swift-DocC Plugin for documentation generation

        * Add a script for publishing docs to GitHub pages

        * Add missing license headers

        * Move README docs to articles in the DocC catalog

        * Remove out-of-date documentation about `DiagnosticEngine`

        Swift markdown no longer includes a DiagnosticEngine so these links were failing
        to resolve.

    commit 5f10cfb030f43222a49ea34857ff07f02f1e38b0
    Author: christopherweems <github@christopherweems.com>
    Date:   Sat Mar 19 12:53:31 2022 -0400

        Fix typo in documentation for `MarkupVisitor` (#26)

        Co-authored-by: Christopher Weems <hello@christopherweems.com>

Package.resolved

@@ -60,7 +60,7 @@
         "repositoryURL": "https://github.com/apple/swift-markdown.git",
         "state": {
           "branch": "main",
-          "revision": "caafc56d3794a08c2203fe417b3aff81e2ab2fc1",
+          "revision": "97df6e2812adcf8698204ca5f0756563ef36e5c1",
           "version": null
         }
       },