accept array of strings for publicDir config

[vitordino]

Body

Summary

Allow more than a single source folder for static assets (publicDir config).

Background & Motivation

I’ve been running into an issue:

1. i’m using “external sources” for images/assets (headless cms, local filesystem outside of the project) to provide content/images/assets.
2. i want the final build version to not include any traces from those sources
3. right now i’m downloading it all the things i need using a astro:config:setup hook
4. i do place them under the /public folder, and build
5. now i have dangling (not git ignored) content laying around on my public dir (not ignored because it has fonts, favicons and other non-cms driven files)

Goals

• Having a way to provide multiple source folders for static files
• Resolve any possible conflict by respecting the order of the array

Example

If the proposal involves a new or changed API, then include a basic code example.
Otherwise, omit this section if it's not applicable.

astro.config.ts

export default defineConfig({
  publicDir: ['public', 'publicGitIgnored']
})

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Similar discussions, but doesn’t solve my specific flow

#434