Add local data.* access and deletion to hx-live

[scriptogre]

Why

Before:

<section data-count="10">
    <button id="save" hx-on:click="
        console.log(
            data.count,            // -> 10 from <section>
            q('#save').data.count  // -> 10 from <section>
        )
    ">Save</button>
</section>

After:

<section data-count="10">
    <button id="save" hx-on:click="
        console.log(
            data.count,            // -> 10 from <section>
            q(this).data.count,       // -> undefined on <button>
            q('#save').data.count  // -> undefined on #save
        )
    ">Save</button>
</section>

Changes

1. Read and write data-* attributes in three ways:

   data.count            // this element, then its ancestors
   q(this).data.count       // this element only
   q('#cart').data.count // #cart only

2. Use delete to remove an attribute:

   delete data.count            // remove the closest data-count
   delete q(this).data.count       // remove data-count from this
   delete q('#cart').data.count // remove data-count from #cart

   null is a stored JSON value:

   data.result = null // write data-result="null"
   data.result        // null

Related to #3928. Either PR can merge first.