[chore] Add automatic alt text for images in CKEditor fields

[superextinct]

This PR adds the retconAutoAlt filter when rendering CKEditor fields, which outputs each asset's alt text on tags.

Note: This filter must be called before any other filters that would serialize the field data. Serialization parses reference tags, preventing Retcon from locating the original asset to retrieve its alt text.

[harryfk]

Does this do something for us, after all?

[superextinct]

Yes! It wasn't working previously in this order:

text                  // <img src="{asset:31382:url||/uploads/Prototypefund-7720.jpg}" />
  |replace({
    '/<div class="col-xs-24">/': "",
    "/<\/div>$/": ""
  })                  // <img src="/uploads/Prototypefund-7720.jpg" />
  |retconAutoAlt      // no bueno, Retcon can't resolve asset from src

It works correctly when retconAutoAlt is called before the CKeditor field data is serialized:

text                  // <img src="{asset:31382:url||/uploads/Prototypefund-7720.jpg}" />
  |retconAutoAlt      // craft\elements\Asset
  |replace({
    '/<div class="col-xs-24">/': "",
    "/<\/div>$/": ""
  })                

[harryfk]

Oh, cool, good to know! That makes a lot of sense, actually.