[astro-dev-only] Make an `import`-able resource that will mark pages to be excluded

[gingerchew]

---
import 'astro-dev-only';
---
<Layout>
  <h1>This page won't show up in prod</h1>
</Layout>

It might make sense to make it a virtual module, so maybe something more like this?

---
import 'dev:only';
---

[essential-randomness]

Oooh I like it. I'm not actually sure about the mechanics of implementing it though, I'll have to think about this as I explore making more integrations.

I feel like it might be less confusing (and possibly more supported) to export something, like export const prerender = true except export const devOnly = true? or similar?

There is also the re-export syntax, though I wonder if people would mistype and import it instead.

export { devOnlyPage } from "astro-dev-only";

[gingerchew]

Are integrations able to look for exports? If so we wouldn't even need to export from the package,

// I would make the argument for just using `export const dev = true` but not a hill worth dying on
export const devOnlyPage = true

I have no idea what event of the integration life cycle would let us check for exports and act on them if there even is one

[gingerchew]

Been having a discussion with some core team members about this here on the astro discord. Looks like export const dev = true won't work, but we could set a file prefix like _dev. to tell Astro to ignore it, then use an integration to inject it, though that is probably more in line with #5 than with this