Fix: JSON Schema Docgen doesn't work on Windows OS

[t-hamano]

Related to #63868

Reported in this comment on Slack

Original report

Since yesterday I am having problems passing the pre-commit check when I use Windows 11. (Yes I have ran npm run distclean and npm ci)
There seems to be something wrong with the paths used in the json-schema-ref-parser or gen-theme-reference.mjs:
I don't think it should be D:\D:\ one should be correct.
stack: 'JSONParserError: Error opening file "D:\D:\LocalSites\67\app\public\wp-content\plugins\gutenberg\schemas\json\theme.json"
ENOENT: no such file or directory (edited)

What?

This PR fixes the docs:theme-ref command so that it works on Windows OS.

Why?

When running the docs:theme-ref command on Windows OS, the following error occurs:

{
  stack: 'JSONParserError: Error opening file "D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json" \n' +
    "ENOENT: no such file or directory, open 'D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json'\n" +
    '    at Object.read (D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\node_modules\\@apidevtools\\json-schema-ref-parser\\dist\\lib\\resolvers\\file.js:61:19)',
  code: 'ERESOLVER',
  name: 'ResolverError',
  message: 'Error opening file "D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json" \n' +
    "ENOENT: no such file or directory, open 'D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json'",
  source: 'D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json',
  path: null,
  toJSON: [Function: toJSON],
  ioErrorCode: 'ENOENT',
  footprint: 'null+D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json+ERESOLVER+Error opening file "D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json" \n' +
    "ENOENT: no such file or directory, open 'D:\\D:\\Desktop\\wp_dev\\WordPress\\gutenberg\\schemas\\json\\theme.json'",
  toString: [Function: toString]
}

When we investigated this issue, we found that THEME_JSON_SCHEMA_URL.pathname has an invalid path like the following:

/D:/Desktop/wp_dev/WordPress/gutenberg/schemas/json/theme.json

I found a similar report about this path in the node project: nodejs/node#37845

How?

According to the comment, this path is a valid URL, but it seems that the URL needs to be converted to a path:

This is the correct output. URLs are not paths. You should convert the url to a path and then perform resolution using that: https://nodejs.org/api/url.html#url_url_fileurltopath_url

Testing Instructions

• Make some changes to the schemas/json/theme.json file.
• Run npm run docs:theme-ref.
• The theme-json-living.md file should be updated.

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: sirreal <jonsurrell@git.wordpress.org>

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[sirreal]

I don't have a windows setup to test, but this makes sense to me 👍