allow sending plugins fn (to get different instances for each router)

[nksaraf]

no other way of cloning plugins without their scope right now

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• infrastructure changes
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What is the current behavior?

What is the new behavior?

Fixes #1195

Right now the user sends a list of plugins in the plugins option. If these plugins depend on keeping some scope that they use to decide what to, there will be issues as we use the same plugins list for multiple router. I want to encourage people to use the fn version of the plugins config, but its a deviation from the default vite API, but only by the slightest and actually provides powerful pattern.

This adds support for the plugins: () => [....] format, while keeping the array format intact.

This was important for example for the imagetools plugin because it adds something to the vite dev server (a middleware) so its needs to be unique for each vinxi router.

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