Evaluate post-summary divider HTML shortcodes

[ptgott]

In HTML content pages with summary dividers, shortcodes after
the summary divider are not evaluated but are simply printed
as-is within the .Content of the page.

The issue takes place in hugolib.newPageContentOutput. In this
function, we apply various transformations to cp.workContent,
and at the end of the function, make this assignment:

cp.content = helpers.BytesToHTML(cp.workContent)

If a page has a manual summary divider and is HTML, we perform
this transformation on cp.workContent:

cp.workContent = src[cp.p.source.posBodyStart:]

While this sets the workContent to all content after the
summary divider, it does so after newPageContentOutput
calls replaceShortcodeTokens and evaluates shortcodes. This
means that evaluated shortcodes are replaced with
unevaluated ones after the summary divider.

The fix is to reuse logic from the function we use to
evaluate the summary divider for non-HTML pages,
hugolib.splitUserDefinedSummaryAndContent. While the current
code branches on whether the page is HTML before calling
this function, this change branches within the function,
since the function already includes a "markup" parameter we
can use to perform the branching.

And since this function already splits the summary/content
while preserving evaluated shortcodes, we can reuse its splitting
logic to fix the issue.

Fixes #6513

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
Please check https://github.com/gohugoio/hugo/blob/master/CONTRIBUTING.md#code-contribution and verify that this code contribution fits with the description. If yes, tell is in a comment.
This PR will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.