Preserve classname

[texodus]

Previously, while subsequent calls to draw() would re-use pre-existing DOM elements such as <th> when appropriate, they would pro-actively clear className before applying the default classes and calling style handlers. This PR removes this behavior, which suppresses some re-renders at the expense of requiring styleListener() implementations to both set and reset any classes they wish to modify (where currently they only need implement set, since draw() would clear the className itself). regular-table's examples all already had this behavior, but this is nonetheless a breaking change.

The old behavior may be trivially implement in a styleListener by resetting className:

table.addStyleListener(() => {
    for (const td of table.querySelectorAll("td, th"}) {
        td.className = "";
        // ...
    }
});