fix(define): correctly replace SSR in dev

[bluwy]

Description

In dev, the define plugin replaces import.meta.env.SSR with config.build.ssr. It should be based on the plugin ssr option instead to be accurate. This PR fixes that.

Additional context

I refactored the __vite__* naming a bit so it looks nicer 😬

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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