Default download/install to ~/.msvc-go-wine when no dir is given

--dest (download) and the positional dir (install) were previously
required, forcing every user to pick and remember a location. Both now
default to a single hidden ~/.msvc-go-wine, matching the convention most
CLI tools use for their own data dir - still overridable for anyone who
wants a different location.

cmd/msvc-go-wine/paths.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// defaultToolchainDir is where `download`/`install` operate when the user
+// doesn't specify a directory: a hidden ~/.msvc-go-wine, so it doesn't
+// clutter a plain `ls ~`.
+func defaultToolchainDir() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".msvc-go-wine"), nil
+}