fix: Context::get_info: Report displayname as "displayname" (w/o underscore)

Otherwise the user thinks that the config key is "display_name" and can't change it using `set
display_name` command.

node/test/test.mjs

@@ -239,7 +239,7 @@ describe('Basic offline Tests', function () {
       'delete_device_after',
       'delete_server_after',
       'deltachat_core_version',
-      'display_name',
+      'displayname',
       'download_limit',
       'e2ee_enabled',
       'entered_account_settings',

src/context.rs

@@ -715,7 +715,7 @@ impl Context {
         );
         res.insert("journal_mode", journal_mode);
         res.insert("blobdir", self.get_blobdir().display().to_string());
-        res.insert("display_name", displayname.unwrap_or_else(|| unset.into()));
+        res.insert("displayname", displayname.unwrap_or_else(|| unset.into()));
         res.insert(
             "selfavatar",
             self.get_config(Config::Selfavatar)