fix(types): mark explicitImportRequired optional and experimental

[sapphi-red]

Description

HMRPayload type in vite/dist/node/index.d.ts and vite/types/hmrPayload.d.ts was different.
The former one is bundled by api-extractor and the latter one is a hand written file.
api-extractor removes @internal properties.

So in vite/types/hmrPayload.d.ts, HMRPayload has explicitImportRequired property and in vite/dist/node/index.d.ts, HMRPayload does not have that property.

Additional context

discord thread

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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 Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[sapphi-red]

/ecosystem-ci run vitest

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
vitest	✅ success

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
astro	✅ success
iles	✅ success
ladle	✅ success
laravel	✅ success
marko	✅ success
nuxt-framework	✅ success
rakkas	✅ success
storybook	❌ failure
svelte	✅ success
vite-plugin-ssr	✅ success
vite-setup-catalogue	✅ success
vitepress	✅ success
vitest	✅ success
windicss	✅ success