Implement File path <-> URL conversion

[File path <-> URL conversion is not standardised](https://github.com/whatwg/fetch/issues/1338). @karwa created a [comprehensive test suite](https://github.com/karwa/swift-url/blob/main/Sources/WebURLTestSupport/TestFilesData/file_url_path_tests.json) and they are proposing [a Swift implementation.](https://github.com/karwa/swift-url/blob/main/Sources/WebURL/WebURL%2BFilePaths.swift) We could collaborate with @karwa in making this more standard.