Add some helper function to build correct URLs

[Omar G.]

I have found myself with the issue that MyBB doesn't provide a way to build core-friendly forum URLs with the correct appended & or ? characters.

If developers want to append parameters to forum URLs built using the get_forum_url() core function they have to figure out the correct character to append first or simply hardcode (in files or templates) the stock forumdisplay.php?fid=x&key=val URLs which ultimately is unfriendly to the core SEO feature.

I like PluginLibrary build_url() helper functions which the core could implement somehow and even use in part of its core.
https://github.com/mybb/mybb/blob/d2f2e6...#L244-L245
https://github.com/mybb/mybb/blob/d2f2e6....php#L1188

Probably even the pagination code.

I apologize if the message lacks clearness, please feel free to write your feedback.