Deprecate relative asset paths, better warnings for asset!() (#3214)

* Deprecate relative asset paths

* fix paths, better warnings in asset parser

packages/manganis/manganis/src/builder.rs

@@ -22,6 +22,16 @@ impl Asset {
         self
     }
 
+    /// Create a new asset but with a relative path
+    ///
+    /// This method is deprecated and will be removed in a future release.
+    #[deprecated(
+        note = "Relative asset!() paths are not supported. Use a path like `/assets/myfile.png` instead of `./assets/myfile.png`"
+    )]
+    pub const fn new_relative(self) -> Self {
+        self
+    }
+
     /// Get the path to the asset
     pub fn path(&self) -> PathBuf {
         PathBuf::from(self.input.to_string())