Headings not available in block type select dropdown when using `createHeadingBlockSpec`

Describe the bug
Usually, when highlighting text in a paragraph block, you are able to change the block type to a heading. When using custom heading config using createHeadingBlockSpec, the option to convert to a heading disappears.

To Reproduce
Expected behaviour:

• Open basic BlockNote example: https://www.blocknotejs.org/examples/basic/minimal
• Enter some text and highlight it
• Open the block type dropdown
• Headings are present

Broken behaviour:

• Open configuring default blocks example: https://www.blocknotejs.org/examples/custom-schema/configuring-blocks
• Highlight some text
• Open the block type dropdown
• Headings are not present

Misc

Minimal example: https://stackblitz.com/edit/github-xbcndn1l?file=src%2FApp.tsx

It seems that in the dropdown:

• The headings all disappear if you specify allowToggleHeadings: true
• The headings will not be filtered if levels: [1, 2, 3] is specified (i.e. you can convert to heading 6)

Start with the configuring default blocks example and compare its block type dropdown with the basic BlockNote example. Read the createHeadingBlockSpec configuration and trace how allowToggleHeadings and levels affect the dropdown. Done means custom heading configurations show the permitted heading options and filter out disallowed levels.