Error if referenced EIPs are not included in requires preamble header

[Pandapip1]

From ethereum/EIP-Bot#69

[Pandapip1]

I believe this is already added.

[SamWilsn]

We never decided if we wanted to implement this. I'm happy to close if you've changed your mind though!

I still believe that a reference does not necessarily imply a "requires" dependency, though maybe we could do a warn-level lint to the effect of:

warning[markdown-requires]: `EIP-1234` mentioned without being included in `requires` preamble field
 --> input.md:50:8
   |
50 | banana [EIP-1234](./eip-1234.md)
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^ mentioned here
   |
  = help: Unless necessary for understanding or implementation, mentioning another proposal can slow standardization.

[Pandapip1]

Oh, I thought you mean referenced in specification/title/description. Those are already implemented.

I would appreciate a warning, though!