Problem

tsup was not properly handling Windows-style paths (with backslashes) in entry points, causing issues for Windows users when specifying input files using backslashes or mixed path separators.

Solution

Added path normalization using the slash utility to convert Windows backslashes to forward slashes in two key areas:

1. Array entries: When entry points are provided as an array, each path is normalized
2. Object entries: When entry points are provided as an object (with custom output names), each path value is normalized

The normalization ensures consistent path handling across platforms by converting Windows backslashes (\) to forward slashes (/) before processing.

Testing

The changes were tested using both Windows-style and mixed path formats, running on macOS to ensure cross-platform compatibility. The test suite includes:

1. windows: backslash in entry - Tests basic backslash handling
2. windows: complex path handling - Tests nested paths with backslashes

   entry: [
     String.raw`src\nested\input.ts`,
     String.raw`src\other\path\test.ts`
   ]

3. windows: object entry with backslashes - Tests object entries with custom names

   entry: {
     'custom-name': String.raw`src\nested\input.ts`
   }

4. windows: mixed path separators - Tests mixed forward/backward slashes

   entry: [
     'src/nested\\input.ts',
     String.raw`src\other/test.ts`
   ]

All Windows path-related tests pass successfully on macOS, confirming that the changes work correctly while maintaining compatibility with Unix-style paths.

Testing Instructions

You can verify these changes by:

1. Running npm test -- -t "windows:" to run the Windows-specific tests
2. Testing with Windows-style paths in your entry points:

   entry: ['src\\input.ts']
   // or
   entry: { 'output': 'src\\input.ts' }

Issue:

closes: #1150