Draft: Add custom allocator support (mimalloc)

[jschwe]

This is a WIP PR to explore adding support for allocations using an allocator provided by the embedder.
In this PR this requirement is going to be somewhat relaxed, and we still add a feature flag for mimalloc, which makes handling the link dependency easier.

Notes:

• -DJS_USE_CUSTOM_ALLOCATOR doesn't seem to be actually useful / maintained, since any custom implementation still need to copy lots of code from Utility.h
• Some code does make assumption that moz_arena_ allocated pointers can be freed / re-alloced by unprefixed free(), this lead to a patch to StringBuffer.h
• The mimalloc header is currently vendored, but hopefully upstream would accept a patch for the DEP_ syntax of passing the include directory to us.
  • An alternative could be if the embedder informs us via an environment variable of where to look for the header file(s). That might be the more generic solution