Output assets to _astro directory

[natemoo-re]

Changes

• Changes output directory from assets/ to _astro
• Better handling for output file naming
• Closes Move /assets folder to /_astro #5745.

Testing

E2E tests added to verify the build output and build.assets configuration

Docs

Guidance wanted!

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Latest commit: c319fc7

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[delucis]

Left some docs suggestions, but looks good! We don’t have any more detailed docs on build output currently but eventually it might make sense to mention this as part of the deployment guides or something. Maybe something like this:

Static assets will be built to /_astro/ by default (configurable using the build.assets option), so that directory can be safely cached immutably if your deployment platform supports it.