fix: allow to omit region name in endregion tag in code snippets

[KermanX]

fix #1867

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[pkg-pr-new]

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create-slidev

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev@1909

@slidev/client

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/client@1909

create-slidev-theme

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev-theme@1909

@slidev/parser

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/parser@1909

@slidev/cli

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/cli@1909

@slidev/types

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/types@1909

commit: 9f506ec