feat: Skip already compiled code in vite plugin

[flex-jonghyen]

Problem

The Vite plugin currently processes all .css.{ts,js,tsx,jsx} files based solely on file extension, without distinguishing between source files and already-compiled output.

This causes issues in monorepo setups where:

1. Package A (e.g., design system) builds its vanilla-extract files
2. Package B (e.g., application) imports from Package A
3. Vite plugin tries to reprocess the already-compiled files from Package A's
4. This fails because the VE compiler cannot process already-compiled files

Solution

Add a simple check in the transform hook to detect already-compiled vanilla-extract files by looking for the signature pattern: import statements containing .vanilla.css

Key pattern:

// Compiled file (should skip)
import './button.css.ts.vanilla.css';

or 

import './button.css.ts.vanilla-somehash.css'

Why Not Other Approaches?

Path-Based Filtering

Q: Since the Vite plugin has access to A's file path during build, why not filter based on path?

A: Path-based filtering would block use-cases where someone intentionally want to transpile Package A's source code during Package B's build (e.g., in monorepo development mode with direct source imports).

Content-based detection only skips files that are already compiled, preserving flexibility for different monorepo workflows.

Renaming Output Files (as suggested in #1568)

Q: Why not use rollup's assetFileNames option to rename compiled files (e.g., .styles.mjs instead of .css.mjs) to avoid the .css.{js,ts} pattern matching?

A: While this workaround can avoid the immediate issue, it doesn't address the root cause: the plugin attempting to reprocess already-compiled files.

This PR aims to fix the fundamental problem rather than working around it.

The file renaming approach remains valid as a temporary workaround, but this fix ensures the plugin correctly handles all scenarios by default.

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Latest commit: 26ad448

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