fix: define searchindex output format alongside its outputs entry

The previous commit added searchindex to [outputs] home but relied on
the imported mod-flexsearch to provide the format definition. Sites
pinning a mod-flexsearch version that predates the searchindex format
fail with "unknown output format searchindex for kind home".

Define the format here, the same pattern netlify and server use even
though mod-csp also defines them. Identical definitions across modules
merge cleanly and decouple Hinode from the mod-flexsearch release order.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: markdumay

config/_default/hugo.toml

@@ -58,6 +58,12 @@ defaultContentLanguageInSubdir = false
     baseName = "_redirects"
     isPlainText = true
     notAlternative = true
+  [outputFormats.searchindex]
+    mediaType = "application/json"
+    baseName = "flexsearch-index"
+    isPlainText = true
+    notAlternative = true
+    permalinkable = true
 
 [mediaTypes."text/netlify"]
 delimiter = ""