Add an option in the astro config build object to set concurrency

[jwickers]

Changes

This makes concurrency optional using the implementation that was removed before. By default still use a concurrency of 1 and does not change anything in the build.

(reopen #4052)

One can set it in their astro.config.mjs:

export default defineConfig({
	// integrations: [...],
	build: {
		concurrency: 8,
	},
	// adapter: ...
});

Signed-off-by: Jeremy Wickersheimer [email protected]

Testing

Tested with a very large build that has >300k pages (mostly all in one URL pattern). Manages to reduce the total build from 6000s to 3000s with a concurrency of 8.

Docs

May need a Doc update to mention the new setting.