Check type of launch config (#2125)

* Check type of launch config

* fix cargo check

* compile when using an explicit platform with other platforms enabled

* fix formatting

* fix overlapping TryIntoConfig implementations

* fix desktop headless tests

---------

Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

examples/custom_html.rs

@@ -4,7 +4,7 @@
 use dioxus::prelude::*;
 
 fn main() {
-    LaunchBuilder::desktop()
+    LaunchBuilder::new()
         .with_cfg(
             dioxus::desktop::Config::new().with_custom_index(
                 r#"