feat: expose default mainFields/conditions

[sapphi-red]

Description

Split out from #18550

[bluwy]

Should we freeze the arrays so it can't be mutated?

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 1592104: Open

suite	result	latest scheduled
astro	❌ failure	❌ failure
histoire	❌ failure	❌ failure
marko	❌ failure	❌ failure
redwoodjs	❌ failure	❌ failure
remix	❌ failure	❌ failure
sveltekit	❌ failure	❌ failure
vike	❌ failure	❌ failure
vite-plugin-react	❌ failure	❌ failure
vite-plugin-svelte	❌ failure	❌ failure
vite-plugin-vue	❌ failure	❌ failure
vitest	❌ failure	❌ failure
waku	❌ failure	❌ failure

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