Shared featured blocks and allowed blocks config across sub blocks of Nav block

[getdave]

All blocks which can be sub blocks of Nav block seem to shared

• allowedBlocks
• featured blocks (see below)

Let's create a canonical const and shared that between all blocks.

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Let's shared this across all these blocks as a standard piece of config

Originally posted by @getdave in #46426 (comment)

[scruffian]

I'm not sure we can do this, featured blocks and allowed blocks are different lists containing different items.

[getdave]

I think perhaps I meant x2 consts? I was advocating for drying up these constants. They may no longer exist 🤷

[scruffian]

Closing this for now.