Surprising behavior on Windows with drive roots in the middle of a path

Consider the following (tested with v1.3.1 tag):

```swift
let fp = FilePath("Sources/E:\\source")
print(fp.isAbsolute) // false
print(fp.isRelative) // true
print(fp.root) // nil
print(Array(fp.components)) // ["Sources", "E:", "source"]

print(FilePath("Sources").appending(fp.components)) // SystemPackage/FilePathParsing.swift:351: Assertion failed
print(FilePath("Sources").pushing(fp)) // Sources\Sources\E:\source

print(FilePath("Sources").appending(FilePath.Component("C:"))) // SystemPackage/FilePathString.swift:299: Fatal error: FilePath.Component must be created from exactly one non-root component
print(FilePath("Sources").appending("C:")) // Sources
```

It's not clear to me that some of these assertions are expected, or even how a seemingly bogus path like `Sources/E:\\source` _should_ be treated by the APIs. Should the FilePath constructor have asserted right away on this path if containing prohibited characters per https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#naming-conventions? Should there be an alternative validating constructor that returns nil or throws in this case?