This is a quick fix to an issue where the types for PluginsConfig don't
match those used in plugins like
[`@tailwindcss/container-queries`](https://github.com/tailwindlabs/tailwindcss-container-queries).

This was caught by TypeScript with [`exactOptionalPropertyTypes`](
https://www.typescriptlang.org/tsconfig/exactOptionalPropertyTypes.html)
enabled, where TypeScript checks if `undefined` can be supplied as a
value for optional types.

I felt that it made more sense to fix this here, as it makes the core
types more flexible, as opposed to each plugin needing to fix this
when/if they hit it.

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Co-authored-by: Philipp Spiess <hello@philippspiess.com>

This PR updates `lilconfig` from v2.1.0 to v3.1.3. Significant
improvements to this package include Windows absolute path support as
well as ESM config files support. This supersedes
#14029 which has fallen
behind and has conflicts with the upstream branch. This is a critical
update for Next.js apps running in development which have dependencies
on packages that use an updated version of `lilconfig`. I understand
that v4 will not be using `lilconfig` but it's an important update for
users on v3.x in the meantime.

---------

Co-authored-by: Jordan Pittman <jordan@cryptica.me>