test: add basic e2e test

test/example/dune

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+(executable
+  (name main))
+
+(cram (deps ./main.exe))

test/example/dune-project

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+(lang dune 3.10)
+
+(generate_opam_files false)
+
+(package 
+  (name mlx-example)
+  (depends dune mlx))
+
+(dialect
+ (name mlx)
+ (implementation
+  (extension mlx)
+  (preprocess
+   (run mlx-pp %{input-file}))))
+

test/example/main.mlx

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+open Printf
+
+type doc = Element  of string * doc list | Text of string
+
+let render d = 
+  let rec render indent = 
+    let indents = List.init indent (fun _ -> "  ") |> String.concat "" in
+    function
+    | Element(name, children) -> 
+      let children = List.map (render (indent + 1)) children in
+      sprintf "%s<%s>\n%s\n%s</%s>" indents name (String.concat "\n" children) indents name
+    | Text s -> sprintf "%s%s" indents s
+  in
+  render 0 d
+
+let div ~children () = Element ("div", children)
+let text s = Text s
+
+let () = 
+  print_endline (render (
+    <div>
+      <div>(text "title")</div>
+      <div>(text "content")</div>
+    </div>
+  ))

test/example/test.t

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+
+  $ ./main.exe
+  <div>
+    <div>
+      title
+    </div>
+    <div>
+      content
+    </div>
+  </div>