Fix issue #1537 "filepath.split_list requires a trailing separator"

Does `make()` with `count + 1` and appends final component (note a
  trailing separator will now result in an empty final component)
Adds test "tests/core/path/filepath/test_core_filepath.odin"

tests/core/build.bat

@@ -54,6 +54,11 @@ echo Running core:math/linalg/glsl tests
 echo ---
 %PATH_TO_ODIN% run math/linalg/glsl %COMMON%
 
+echo ---
+echo Running core:path/filepath tests
+echo ---
+%PATH_TO_ODIN% run path/filepath %COMMON%
+
 echo ---
 echo Running core:reflect tests
 echo ---