Build Error On Windows

[JinweiOS]

When I built this project on win32 platform by commond npm run update-index, I found an error that said index_data directory can't find.

After debuging, I found these codes const __dirname = new URL('.', import.meta.url).pathname; in scripts of building that caused the error, for pathname is a uri that has a / at the beginning of the string.

This is right on unix-like system, but on windows the file strings look like \C:\Users\Administrator\project\rescript-lang.org\index_data\latest_belt_api_index.json is bad, it should start with C:.......

So I initiated this PR

[JinweiOS]

The PR has been merged!