feat: enable extending user tsconfig

[robBowes]

Closes https://github.com/sveltejs/kit/issues/6868 by allowing the user to specify a root tsconfig to extend. This is useful for monorepos where the is a base tsconfig file at the root that needs to be extended.

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[Rich-Harris]

Thank you. extends probably isn't the right name — it should be something tsconfig-specific.

I wonder if a more future-proof approach might be to allow the generated config to be augmented in arbitrary ways? Something like this:

// svelte.config.js
import path from 'path';

export const config = {
  kit: {
    tsconfig: (tsconfig) => {
      tsconfig.extends = path.resolve('../tsconfig.json');
    }
  }
};

Or this:

// svelte.config.js
import path from 'path';

export const config = {
  kit: {
    tsconfig: (tsconfig) => {
      return {
        ...tsconfig,
        extends: path.resolve('../tsconfig.json')
      };
    }
  }
};

I don't know if tsconfig is the best name for the option.

[dummdidumm]

I like the second version of Rich's proposal. I think tsconfig is the right name.

[Rich-Harris]

going to close this in favour of #8606 which implements the idea above — thanks!