Make heap size hint available as an env variable (#55631)
This makes `JULIA_HEAP_SIZE_HINT` the environment variable version of
the `--heap-size-hint` command-line flag. Seems like there was interest
in
https://github.com/JuliaLang/julia/pull/45369#issuecomment-1544204022.
The same syntax is used as for the command-line version with, for
example, `2G` => 2 GB and `200M` => 200 MB.
@oscardssmith want to take a look?
