Use which to resolve lsp/dap binaries

This resolves the following issue: https://github.com/helix-editor/helix/discussions/962#discussioncomment-1580046

helix-dap/src/client.rs

@@ -105,6 +105,9 @@ impl Client {
         args: Vec<&str>,
         id: usize,
     ) -> Result<(Self, UnboundedReceiver<Payload>)> {
+        // Resolve path to the binary
+        let cmd = which::which(cmd).map_err(|err| anyhow::anyhow!(err))?;
+
         let process = Command::new(cmd)
             .args(args)
             .stdin(Stdio::piped())