fix(plugin-vue): allow to overwrite feature flags

[fahchen]

Description

Feature flags like __VUE_OPTIONS_API__ can't be overwritten, It seems to have been deleted in this commit a93ab23#diff-2b066c2eb45a74f1191f8796c6c76a93a7bc81417b11b8852e965aec86be2cc4 , this PR adds it back.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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