Exclude generated lexicon files from linting and formatting

Add src/lexicon to biome ignore list since these files are
auto-generated by @atproto/lex-cli and should not be linted
or formatted.

Co-Authored-By: Claude Sonnet 4.5 <[e-mail]>

biome.json

···
   },
   "files": {
     "ignoreUnknown": false,
-    "ignore": ["node_modules", "dist", "coverage"]
+    "ignore": ["node_modules", "dist", "coverage", "src/lexicon/**", "lexicons/**"]
   },
   "formatter": {
     "enabled": true,