fix: rename type files as lowercase (fixes #4028)

[IanVS]

Description

This changes the files in vite/types to all be lowercase, so that they can be referenced in typescript triple-slash directives, which for some reason seem to always lowercase the provided path.

Additional context

I think it's likely a bug in typescript, but I don't think we can hold our breath on a change upstream, and this doesn't seem like a horrible workaround.

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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.
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
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I couldn't find a good way to write tests for this, but I did build and copy the results into my node_modules of my project, and verified that it solves the issue I was having.

[IanVS]

Replaced by #4031