Add path validation utility and integrate with CLI

src/util/fs.rs

@@ -0,0 +1,6 @@
+use std::path::PathBuf;
+
+pub fn path_exists(s: &str) -> Result<PathBuf, String> {
+    let p = PathBuf::from(s);
+    p.exists().then_some(p).ok_or("path does not exist".to_string())
+}