Update dependency vue-i18n to v9.1.8

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
vue-i18n	9.1.7 -> 9.1.8

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Release Notes

intlify/vue-i18n-next

### [`v9.1.8`](https://togithub.com/intlify/vue-i18n-next/releases/v9.1.8) [Compare Source](https://togithub.com/intlify/vue-i18n-next/compare/v9.1.7...v9.1.8) #### v9.1.8 (2021-10-04) ##### :bug: Bug Fixes - `message-compiler`, `vue-i18n` - [#​720](https://togithub.com/intlify/vue-i18n-next/pull/720) fix: bring bug fixes from v9.2.0-beta.11 ([@​kazupon](https://togithub.com/kazupon)) ##### Committers: 1 - kazuya kawaguchi ([@​kazupon](https://togithub.com/kazupon))

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