It's easily possible for Lua to raise errors where the string is not
valid UTF-8. When we turn that into a Scribunto_LuaError, we should
normalize it so other things don't break.
Bug: T208689
Change-Id: Idc5514261e99d64222b86877dd0500d425a26988
Manually import LuaSandbox's git repository as a composer dependency to
provide the PHP stubs for phan.
Change-Id: I6226b9211f31d829da5a2775c6f5cf3599dd8ebc