Non-Letter Drive Windows Paths Are Incorrectly Reported As Non-Absolute

This is an edge-case for sure.

Here’s the source code:
```swift
print(FilePath(#"C:\foo"#).isAbsolute)
print(FilePath(#"+:\foo"#).isAbsolute)
print(FilePath(#"€:\foo"#).isAbsolute)
```

The three paths are valid absolute paths on Windows, however the output of this program will be `true`, `false`, `false`.

For more info see .

Start by locating the FilePath.isAbsolute implementation and its Windows path handling, then reproduce the three examples from the issue. The work is done when C:\foo, +:\foo, and €:\foo are all reported as absolute paths on Windows, with tests covering the behavior.

Written by the indexing model from the issue text.