Fix URL file:// path handling on Windows

[colinrotherham]

This PR fixes two Windows regressions added in #3527 and #3372

URL file paths with drive letters D:/ confuse path.relative() and throw errors in await import():

Error [ERR_UNSUPPORTED_ESM_URL_SCHEME]: Only URLs with a scheme in: file and data are supported by the default ESM loader. On Windows, absolute paths must be valid file:// URLs. Received protocol 'd:'

File URLs with absolute paths

On Windows, Node.js file:// URL paths look like:

// Example file URL string
const file = 'file:///D:/path/to/asset'

Converting URLs to paths incorrectly

But the property new URL(file).pathname includes a leading forward slash:

// Example file URL pathname ❌
const pathname = '/D:/path/to/asset' 

We hadn't realised this breaks path.relative() on Windows with paths joined and not resolved:

'..\\..\\..\\..\\D:\\path\\to\\project\\packages\\govuk-frontend\\src\\govuk\\components\\character-count\\_index.scss'

Converting URLs to paths correctly

After investigating this issue (also "Converting between URLs and file paths") we see that:

1. Compatible cross-platform “path to file URL” changes must use pathToFileURL()
2. Compatible cross-platform “file URL to path” changes must use fileURLToPath()

// Example file URL via fileURLToPath() ✅ 
const path = 'D:\\path\\to\\asset'

[colinrotherham]

Test runs on Windows must be run manually:
https://github.com/alphagov/govuk-frontend/actions/runs/4893403132

[colinrotherham]

Had to move this line here for Windows to find node_modules/.bin/tsc

Unlike our Linux runners it's not in the global “npm get cache” location at ~/.npm

> build:types
> tsc --build tsconfig.build.json

'tsc' is not recognized as an internal or external command,
operable program or batch file.

[romaricpascal]

Cheers for catching that! Looks good to me ⛵