Ability to exclude files from compilation

Describe the problem

There are times when we need to exclude files from compilation. Here's some examples:

• An app might not have some routes in the PWA build, like /admin, but it might have them in the SSR build
• Some files can be problematic during compilations, like +page.server.ts

At the moment, these files have to be manually removed before the build then put back afterwards. It's cumbersome.

Describe the proposed solution

In svelte.config.js, define a function that acts as a filter to source code files so we can ignore some files.

Alternatives considered

Right now, if you want to ignore some files, you have to straight up delete them.

find src/routes -name '+*.server.*' -delete
pnpm build
http-server ./build
git checkout -- 'src/routes/+*.server.*'

Importance

would make my life easier

Additional Information

• Fixes https://github.com/sveltejs/kit/issues/10332
• https://github.com/sveltejs/kit/issues/6031#issuecomment-1593328625

Start with the proposed configuration in svelte.config.js and trace how source files under src/routes are selected during pnpm build. Check how files such as +page.server.ts are handled, then define the filter behavior so selected files can be excluded without deleting them; done means the build honors the configured exclusions.