Use mediawiki/at-ease library for suppressing warnings

Use mediawiki/at-ease library for suppressing warnings

wfSuppressWarnings() and wfRestoreWarnings() were split out into a
separate library. All usages in core were replaced with the new
functions, and the wf* global functions are marked as deprecated.

Additionally, some uses of @ were replaced due to composer's autoloader
being loaded even earlier.

Ie1234f8c12693408de9b94bf6f84480a90bd4f8e adds the library to
mediawiki/vendor.

Bug: T100923
Change-Id: I5c35079a0a656180852be0ae6b1262d40f6534c4