chore(all): 1.9.1 changelog

Author: ulivz

Diff for: CHANGELOG.md

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+## [1.9.1](https://github.com/vuejs/vuepress/compare/v1.9.0...v1.9.1) (2021-12-22)
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+### Bug Fixes
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+* **types:** support plain string usage for known third-party plugins ([dd6e3ef](https://github.com/vuejs/vuepress/commit/dd6e3ef)) [@chenhaoli](https://github.com/chenhaoli)
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 # [1.9.0](https://github.com/vuejs/vuepress/compare/v1.8.3...v1.9.0) (2021-12-22)