Fix generated shell scripts for Cygwin/MSYS2/MinGW

[500-internal-server-error]

Reference: npm#30

This PR is based on the PR above, and also fixes the NODE_PATH pnpm additionally seems to rely on that npm does not. pnpm run test completes with no issues, though I am unsure how to build the entirety of pnpm with these changes for further testing.

[Copilot]

Pull Request Overview

This PR addresses issues with generating shell script shims on Cygwin, MSYS2, and MinGW environments, and also updates the NODE_PATH normalization logic required by pnpm.

• Introduces an isCygwin helper function for environment detection.
• Updates shell shim generation to handle MINGW and MSYS conditions.
• Adjusts the NODE_PATH normalization to differentiate between Cygwin (/proc/cygdrive) and Windows (/mnt) environments.

Files not reviewed (2)

• test/snapshots/e2e.test.js.snap: Language not supported
• test/snapshots/test.js.snap: Language not supported

Comments suppressed due to low confidence (1)

src/index.ts:675

• Verify that using '/proc/cygdrive' for Cygwin environments is consistent with all use cases and does not lead to unexpected path resolution issues, especially compared to the default '/mnt' behavior on Windows.

const posix = isWindows() ? split[i].split('\').join('/').replace(/^([^:\/]*):/, (_, $1) => `${isCygwin() ? '/proc/cygdrive' : '/mnt'}/${$1.toLowerCase()}`) : split[i]