Allow normalisation and/or validation of categories from listings

[mcanouil]

From Quarto office hours:

In listings, it is easy to misspell categories or apply inconsistent capitalisation, such as mixing lower case, upper case, or other variations. This issue becomes particularly common when multiple contributors are involved.
Introducing a mechanism to standardise and/or validate categories against a reference file (perhaps in YAML format) would be highly beneficial.

[mcanouil]

Here is a very simplified version of what I was talking about using a custom listing to normalise the case of categories:

```{=html}
<div class="list quarto-listing-default">
  <% for (const item of items) { %>
    <%
    if (item.categories && Array.isArray(item.categories)) {
      item.categories = item.categories.map(category => category.toUpperCase());
    }
    %>
    <div class="quarto-post" data-analysis-status="<%= item.status %>" <%= metadataAttrs(item) %>>
      <div class="body">
        <a href="<%- item.path %>">
          <h3 class="no-anchor listing-title">
            <%= item.title %>
          </h3>
        </a>
        <div class="listing-categories">
          <% for (const category of item.categories) { %>
            <div class="listing-category" onclick="window.quartoListingCategory('<%= utils.b64encode(category) %>'); return false;"><%= category %></div>
          <% } %>
        </div>
      </div>
    </div>
  <% } %>
</div>
```