Simplify GC fixallcollectable() logic and move to a separate file

This also removes a lot of the nasty special-cases we used to have
to maintain, and moves things to a proper depth-first graph traversal
that can handle reference cycles.

My god, it's all graphs.

Author: HaroldCindy

Sources.cmake

@@ -368,6 +368,7 @@ target_sources(Luau.VM PRIVATE
     VM/src/lfunc.cpp
     VM/src/lgc.cpp
     VM/src/lgcdebug.cpp
+    VM/src/lgcfix.cpp
     VM/src/lgcgraph.cpp
     VM/src/lgctraverse.cpp
     VM/src/linit.cpp