Configure output directory (currently fixed to ".svelte-kit")

Describe the problem

In monorepos, builds are often centralized into a single top-level build directory.

Is there anything that prevents that change from being innocent and simple?

Describe the proposed solution

In order for svelte-kit to support that, the SVELTE_KIT constant in constants.js should be replaced by a configuration option.

setting the kit.vite.build.outDir options prints an error:

build_server: The value for kit.vite.build.outDir specified in svelte.config.js has been ignored. This option is controlled by SvelteKit.

should the vite option be used, or should there be svelte-kit specific option?

Importance

nice to have

[Rich-Harris]

The directory is used for more than Vite builds, so we can’t use vite.build.outDir — Kit needs to control it. Having it be configurable seems reasonable, we would just need to bikeshed a name. buildDir? outDir? tmpDir? something else?

[Rich-Harris]

I did, yeah — closed by accident. An outDir outside the project directory should work as long as you don’t specify paths config, but we need to address that case before we can consider this fully closed