Proposed syntax: creator:Foo and creator:#123

Use case:

1. A bot created a large number of articles, and we want to find search results not created by this bot.
2. I want to work on articles in a category, but I want to skip articles created by User:Example.
3. I want to find articles created by some user that is less recently edited.

Note:

1. Internally we only store user ID for registered users, so there will be no disruption on user rename
2. Delete/Undelete/Revdel/Import may affect the data (no creator may be exposed if it is revdeled)
3. UserMerge may disrupt existing data but it is not enabled in Wikimedia wikis