[sigma] Fixes #1425

The built declarations for edge-arrow/index.ts were somehow broken, with
imports starting with "packages/sigma/types/...", which could not be
resolved, especially within Angular applications.

I could not understand exactly why that type was wrongly generated, but
I could solve the issue by explicitely setting the type for
EdgeArrowProgram.

packages/sigma/src/rendering/programs/edge-arrow/index.ts

@@ -5,10 +5,10 @@
  * Compound program rendering edges as an arrow from the source to the target.
  * @module
  */
-import { createEdgeCompoundProgram } from "../../edge";
+import { EdgeProgramType, createEdgeCompoundProgram } from "../../edge";
 import EdgeArrowHeadProgram from "../edge-arrow-head";
 import EdgeClampedProgram from "../edge-clamped";
 
-const EdgeArrowProgram = createEdgeCompoundProgram([EdgeClampedProgram, EdgeArrowHeadProgram]);
+const EdgeArrowProgram: EdgeProgramType = createEdgeCompoundProgram([EdgeClampedProgram, EdgeArrowHeadProgram]);
 
 export default EdgeArrowProgram;