Broken autoformat / markup shortcuts for headings in Slate text block

[mbarde]

Describe the bug

Markup shortcuts for headings (# for h2 or ## for h3) in slate text blocks do not work (anymore).

To Reproduce

• Go to https://demo.plone.org (or any other page using Volto 17.x)
• Edit a page
• Add text block
• Enter # new heading
• Text still formatted as normal text, but heading icon is selected in toolbar (see screenshot)

JSON of block looks like this:

{
  "@type": "slate",
  "plaintext": "heading",
  "value": [
   {
    "type": "p",
    "children": [
     {
      "text": "heading",
      "type": "h2"
     }
    ]
   }
  ]
 }

Expected behavior

Text should be formatted as heading.

JSON should like that (as opposed to above snippet):

{
  "@type": "slate",
  "plaintext": "heading",
  "value": [
   {
    "type": "h2",
    "children": [
     {
      "text": "heading",
      "type": "h2"
     }
    ]
   }
  ]
 }

Screenshots

Software (please complete the following information):

• Plone Version: 6.0.8
• Plone REST API Version: 9.1.2
• Volto Version: 17.6.0
• OS: Ubuntu 22.04
• Browser: Firefox, Google Chrome

[pranayjoshi]

The case is same in the case of Italics and every other commands