feat(turbopack): support merged PostCSS configs

[fireairforce]

Summary

• discover a PostCSS config file even when inline config content is supplied
• append inline plugins after file plugins in a single PostCSS pass
• keep the existing runtime postcss.ts implementation unchanged
• preserve config-file invalidation and the existing file-only/inline-only paths

Motivation

Utoopack integrations need to inject framework PostCSS plugins without suppressing a user-owned postcss.config.* file. Previously inline config short-circuited config discovery, so the two sources were mutually exclusive.

Implementation

The change is isolated to Rust. File-only and inline-only configurations continue through the original executor path. When both sources exist, Rust creates a lightweight virtual config module that resolves the two configs sequentially and concatenates their plugin declarations, then passes that single config to the unchanged runtime executor.

Downstream

• feat(pack): support merge postcss config utoo#3320

Verification

• cargo fmt --all and git diff --check
• cargo check -p turbopack-node -p turbopack --lib
• cargo clippy -p turbopack-node -p turbopack --lib -- -D warnings
• downstream Utoopack snapshots cover file-only, inline-only, and merged configurations