[TS] Fix relative paths for exports (#8517)

Fixes an issue where exports were using incorrect relative paths for
>=3 namespace levels. This is fixed by making the starting range of the
namespace components relative to the amount of components.

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>

tests/longer_namespace.fbs

@@ -0,0 +1,7 @@
+namespace longer_namespace.a.b.c;
+
+table Person {
+  name:string;
+  age:short;
+}
+root_type Person;