[whatwg/fetch] Define ABNF for `origin-or-null. (PR #1924)

The ABNF for `Access-Control-Allow-Origin` relies upon the `origin-or-null` syntax, which doesn't exist in RFC6454 (see https://www.rfc-editor.org/errata/eid3249). This patch defines the intended syntax.

This is intended to be an editorial change without any change in behavior, but adding grammar is somewhat normative? Idunno.

This patch also takes a normative dependency on RFC6454 for the `serialized-origin` grammar, which might not be ideal. We could copy it here if you'd prefer.

WDYT?


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-- Commit Summary --

  * Define ABNF for `origin-or-null.

-- File Changes --

    M fetch.bs (9)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1924.patch?email_source=notifications&email_token=ACLYVMPAZLW4WPDXKMLWNRL4W4RNBA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTKNRQGQ4DGMJYG2THEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVZYHEX3QMF2GG2C7MNWGSY3Lhttps://github.com/whatwg/fetch/pull/1924.diff?email_source=notifications&email_token=ACLYVMMMKZ3IPVDPISRDD2L4W4RNBA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTKNRQGQ4DGMJYG2THEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVVYHEX3ENFTGMX3DNRUWG2Y
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