Support CDDL dfns, extract standard prelude from CDDL RFC

[tidoust]

New types of definitions will soon be introduced in Bikeshed to deal with CDDL blocks: cddl-module, cddl-type, cddl-parameter, cddl-key, cddl-value. The extraction now considers that these definition types are valid.

To be able to reference CDDL types defined in the standard prelude, a pre-processing step is also added for the CDDL RFC (RFC8610). To avoid creating multiple definitions with the same id and href, which could perhaps confuse consuming tools (?), all types defined in the standard prelude get attached as different linking text to the same cddl-type dfn, leading to the following dfns extract:

[
  {
    "id": "appendix-D",
    "href": "https://www.rfc-editor.org/rfc/rfc8610#appendix-D",
    "linkingText": [
      "any",
      "uint",
      "nint",
      "int",
      "bstr",
      "bytes",
      "tstr",
      "text",
      "tdate",
      "time",
      "number",
      "biguint",
      "bignint",
      "bigint",
      "integer",
      "unsigned",
      "decfrac",
      "bigfloat",
      "eb64url",
      "eb64legacy",
      "eb16",
      "encoded-cbor",
      "uri",
      "b64url",
      "b64legacy",
      "regexp",
      "mime-message",
      "cbor-any",
      "float16",
      "float32",
      "float64",
      "float16-32",
      "float32-64",
      "float",
      "false",
      "true",
      "bool",
      "nil",
      "null",
      "undefined"
    ],
    "localLinkingText": [],
    "type": "cddl-type",
    "for": [],
    "access": "public",
    "informative": false,
    "heading": {
      "href": "https://www.rfc-editor.org/rfc/rfc8610",
      "title": "RFC 8610: Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures"
    },
    "definedIn": "pre"
  }
]

I note it would be easy to create one definition per CDDL type otherwise... but then they would have the same id and same href. In practice, I do not know whether that would be a problem for anyone (we don't guarantee that dfns have unique IDs per se, although that should be the case unless a spec incorrectly reuses an ID in more than one dfn).