set explicit encoding when reading and writing hex config

[blandinw]

(fixes erlef/rebar3_hex#184)

The issue was that the bytes were written "as is" on disk, while Erlang source files are expected to be utf8 (and no longer latin1) by default since Erlang/OTP 17.0 (as per https://erlang.org/doc/reference_manual/character_set.html)
e.g. byte <<200>> (0xC8) is not valid utf8, but was written as is, causing file:consult to raise an encoding error.

It is preferable not to rely on default behavior, and specify an encoding both at write and read time.
This PR specifies utf8 (probably more future-proof than latin1) when calling file:write_file and adds the encoding at the top of the file for future reads via file:consult (as per https://erlang.org/doc/reference_manual/character_set.html#source-file-encoding).

The test writes to the actual config at the moment. How should I stub it out, knowing that it should still use file:file_write and file:consult and have the same semantics? Maybe an in-memory file, etc

cc @starbelly @ferd