fix: add isochrone to cspell word list

cspell doesn't know 'isochrone' by default. Adding both cases to the
project word list so the CI spellcheck passes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mattpodwysocki

cspell.json

@@ -21,6 +21,8 @@
     "Tilesets",
     "tilestats",
     "Tilequery",
+    "isochrone",
+    "Isochrone",
     "aerialway",
     "aeroway",
     "housenum",