custom aria role description for blocks

[maribethb]

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Problem

we set the aria role-description for blocks as "value blocks" or "command blocks" or similar here

but developers may want to use custom values for this, such as "boolean block"

Request

add methods to Block to facilitate setting the role description from the block definition in both js and json.

more details can be added here from the design doc when available.

Alternatives considered

No response

Additional context

No response