fix: explicitly export ssrRegisterHelper as const in plugin-vue-jsx (fix #376)

[luckydye]

Description

Fixes #376

"The Bun implementation of Function.toString differs from node, as described in oven-sh/bun#9543.
This causes the serialization of ssrRegisterHelper to output a incorrect string."

This change exports a const for the named export of the ssrRegisterHelper to make sure the name for the exported function exists. Will otherwise result in a parse error.

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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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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[edison1105]

LGTM

[pkg-pr-new]

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pnpm add https://pkg.pr.new/@vitejs/plugin-vue@380

pnpm add https://pkg.pr.new/@vitejs/plugin-vue-jsx@380

commit: ea224cb