[twitter] add 'limit' option (#8173)

- reduce default number of requested items to 50
- allow using "fallback" values like with 'search-limit'

https://github.com/mikf/gallery-dl/issues/8173#issuecomment-3303015837

docs/gallery-dl.conf

@@ -871,6 +871,7 @@
             "cursor"      : true,
             "expand"      : false,
             "include"     : ["timeline"],
+            "limit"       : 50,
             "locked"      : "abort",
             "logout"      : true,
             "pinned"      : false,