RelayMark block struct and helpers.
@type t() :: %RelayMark.Block{ content: [t() | RelayMark.Inline.t()], evidence_refs: [String.t()], id: String.t(), props: map(), type: String.t() }
@spec from_map(map()) :: t()
@spec plain_text(t() | RelayMark.Inline.t()) :: String.t()