fix: probe for an executable at a relative dprint.path on Windows

[dsherret]

Fixes a dprint.path setting like .vscode/dprint no longer working on Windows when there's a .vscode/dprint.bat beside a .vscode/dprint shell script for other platforms (see dprint/dprint#859 (comment)). This regressed in #163 when dprint started being launched without cmd.exe where possible.

• A non-absolute dprint.path containing a path separator (ex. .vscode/dprint or bin\dprint) is now resolved against the folder on Windows. Previously only paths starting with ./ or ../ were, so others were searched for relative to the extension host's cwd.
• For a file path without an executable extension, the launchable PATHEXT extensions (.exe, .com, .cmd, .bat) are now probed before the path itself, like cmd.exe does. Previously the extensionless file won when it existed, which can't be launched when it's a shell script. This also helps $(yarn bin dprint), which resolves to a node_modules/.bin/dprint script beside a dprint.cmd.

Command names such as dprint are still searched for on the path as before and nothing changes on macOS and Linux.