Correctly normalize UNC paths

Add support for normalization of UNC paths in the filesystem. `_normalizePath` is now a method of the `FileSystem` class. Its behavior w.r.t. UNC paths is controlled by a new `FileSystemImpl` flag, `normalizeUNCPaths`. If set, paths like `///server//foo/bar` are normalized to `//server/foo/bar/` instead of `/server/foo/bar/`. `AppshellFileSystemImpl.normalizeUNCPaths` is set at runtime for Windows only.

Addresses #6028 and #6029.

CC @gruehle @peterflynn