Cleanup: move mesh tessellation into it's own file

This matches BMesh which also has tessellation in it's own file.

Using a separate file helps with organization when
extracting code into smaller functions.

source/blender/bmesh/intern/bmesh_mesh_tessellate.c

@@ -19,6 +19,8 @@
  *
  * This file contains code for polygon tessellation
  * (creating triangles from polygons).
+ *
+ * \see mesh_tessellate.c for the #Mesh equivalent of this file.
  */
 
 #include "DNA_meshdata_types.h"