fix(preload): whitelist get-pdf-form-fields and pdf-form-fields channels

The get-pdf-form-fields IPC pair (handler in main.js, renderer invoke and
event.reply('pdf-form-fields')) was missing from ALLOWED_SEND_CHANNELS and
ALLOWED_RECEIVE_CHANNELS. Under the planned preload migration an unlisted
channel is silently blocked, so the form-field fill/flatten feature would
break once the main window stops using the inline shim. Placed adjacent to
the sibling get-pdf-page-count/pdf-page-count pair it was modeled on.

Amit Haridas

src/preload.js

@@ -86,6 +86,7 @@ const ALLOWED_SEND_CHANNELS = [
   // PDF operations
   'process-pdf-operation',
   'get-pdf-page-count',
+  'get-pdf-form-fields',
   'get-pdf-capabilities',
   'select-pdf-folder',
   'batch-pdf-operation',
@@ -235,6 +236,7 @@ const ALLOWED_RECEIVE_CHANNELS = [
 
   // PDF operations
   'pdf-page-count',
+  'pdf-form-fields',
   'pdf-operation-complete',
   'pdf-operation-error',
   'pdf-batch-complete',