Refactor PPX: move across files and clean up exceptions

Move PPX V3 and V4 back to different files.
Clean up treatment of exceptions.

Author: cristianoc

cli/dune

@@ -1,7 +1,8 @@
 (executable
  (name res_cli)
  (public_name rescript)
+ (modes byte exe)
  (flags
   (-open Syntax -open Compilerlibs406)
-  (:standard -w +a-4-42-40-9-48))
+  (:standard -w +a-4-42-40-9-48-70))
  (libraries syntax compilerlibs406))

cli/react_jsx_common.ml

@@ -0,0 +1,24 @@
+open Asttypes
+open Parsetree
+
+type jsxConfig = {
+  mutable version: int;
+  mutable module_: string;
+  mutable mode: string;
+  mutable nestedModules: string list;
+  mutable hasReactComponent: bool;
+}
+
+(* Helper method to look up the [@react.component] attribute *)
+let hasAttr (loc, _) = loc.txt = "react.component"
+
+(* Iterate over the attributes and try to find the [@react.component] attribute *)
+let hasAttrOnBinding {pvb_attributes} =
+  List.find_opt hasAttr pvb_attributes <> None
+
+let raiseError ~loc msg = Location.raise_errorf ~loc msg
+
+let raiseErrorMultipleReactComponent ~loc =
+  raiseError ~loc
+    "Only one component definition is allowed for each module. Move to a \
+     submodule or other file if necessary."