Build/Test Tools: Instruct Git and Subversion to ignore Composer's `/vendor` directory.

Re-adds changes from [42405] after being reverted by accident in [43309].

Fixes #45770

Built from https://develop.svn.wordpress.org/trunk@44363


git-svn-id: https://core.svn.wordpress.org/trunk@44193 1a063a9b-81f0-0310-95a4-ce76da25c4cd