feat: allow duplicated keys behind an option

[kovsu]

refs: prettier/prettier#18407
cc @fisker

[fisker]

Let's write some tests, something like

const tests = [] // From `yaml-test-suite` or write some directly

for (const ... of tests) {
  expect(() => parse(''))....(); // assert error
  expect(() => parse('', {allowDuplicatedKeysInMap: true}))....(); // assert success
}

[fisker]

I think {a:1, a:2} is a different map?

[fisker]

Released https://github.com/prettier/yaml-unist-parser/blob/main/changelog.md#220-2025-12-03

[fisker]

Changed the name to allowDuplicateKeysInMap 737e2b3