feat(DeadLinks): add config override for resolving custom link markDefs

[aliameur]

This PR proposes customisation for the DeadLinks plugin to solve #2

Example usage

Simple override

Preflight({
  plugins: [
    DeadLinks({
      content: "content",
      overrideLinkKeys: ["url", "href"],
    }),
  ],
})

A slightly more advanced override

// these types could be generated with sanity typegen generate
type MyExternalLinkType = {
  url: string;
  target: string;
}

type MyProductLinkType = {
  fullUrl: string;
  showOnHover?: boolean;
}

type MyCustomLinkType = {
  hrefForAdmin: string;
  hrefForGuest: string;
}

Preflight({
  plugins: [
    DeadLinks<MyExternalLinkType | MyProductLinkType | MyCustomLinkType>({
      content: "content",
      overrideLinkKeys: ["url", "fullUrl", "hrefForGuest"], // typescript will only allow keys of the generics above
    }),
  ],
})

Questions

• Should this directly support doing typescript typeof mySchemaType? This adds a lot more complexity that at first thought may fall outside the scope of library, but admittedly a nice DX.
• Are there any other ways links could be embedded in portable text that this proposed override would not pick up?
• How should an example be portrayed in the README.md?