Add pre and post fields for overriding ToC heading

[IveGotNorto]

Pre and post allow users to add semantically correct headings to ToC
without breaking existing functionality.

See #8338

[CLAassistant]

All committers have signed the CLA.

[jmooring]

@IveGotNorto

I pulled this locally for some quick testing. Comments...

• If pre is not defined in the site configuration, its value should be <nav id="TableOfContents">

• If post is not defined in the site configuration, its value should be </nav>

• I should be able to override pre independently of post. For example:

  [markup]
    [markup.tableOfContents]
      pre = "<nav><h1>Navigation</h1>"

• I should be able to override post independently of pre. For example:

  [markup]
    [markup.tableOfContents]
      post = "</nav><!-- end of nav -->"

• If pre = "" in the site configuration, the outer <ol> or <ul> should not be preceded by anything (i.e., do not insert <nav id="TableOfContents">).

• If post = "" in the site configuration, the outer </ol> or </ul> should not be followed by anything (i.e., do not insert </nav>).

[jmooring]

@IveGotNorto Thank you for working on this!

[jmooring]

@IveGotNorto

Although this is working as intended, the defaults for pre and post should not be nil.
https://github.com/gohugoio/hugo/pull/8643/files#diff-be327cc7b6df56b7b25c4775d78955598913504707bd422132aec74e1b5ffc39R169-R170

The defaults should be <nav id="TableOfContents"> and </nav>.

[bep]

See #8670

[IveGotNorto]

Not really sure where this PR stands. Should I be making some different changes related to #8670? @bep @jmooring

[jmooring]

I wanted to revist this today to test your changes, but it no longer builds due to a7e3da2.

I cannot answer your question about where this PR stands in relation to #8670 and #8680. It seems like we're in a holding pattern.

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