feat(deno.jsonc): Add keywords to package

This commit adds the keywords "svelte", "preprocess", "css", and "import"
to the deno.jsonc file. These keywords will help improve the visibility
of the package in searches.

Author: ryoppippi

deno.jsonc

@@ -37,5 +37,10 @@
       ".coverage"
     ]
   },
-  "keywords": []
+  "keywords": [
+    "svelte",
+    "preprocess",
+    "css",
+    "import"
+  ]
 }