Fix node-gyp-build interop for CJS importers in ESM output

[vweevers]

Scenario: I'm consuming a native addon from a CommonJS npm package, in an ESM bundle. Problem: the vite-plugin-native-modules plugin rewrote require('node-gyp-build')(__dirname) to a require() that points at the plugin's ESM virtual native wrapper. That made the CJS package receive the namespace object instead of the native addon itself.

To make vite-plugin-native-modules work in that scenario, I had to change the addon's entry point from:

const addon = require('node-gyp-build')(__dirname)

To:

const addonWrapper = require('node-gyp-build')(__dirname)
const addon = addonWrapper.default || addonWrapper

This PR fixes it in the plugin. What I changed:

• In src/index.ts, CJS native-loader rewrites now emit .default when the final output format is ESM.
• Added regression coverage in test/node-gyp-build.test.ts.
• Fixed two Windows issues while there:
  • Normalized \ paths for additionalNativeFiles (a6ed11c)
  • Relaxed test regexes so that win32 is valid (21d25af).