build: consider using `--mm:orc`

[ee7]

This also sets --exceptions:goto.

[ee7]

Looks like --gc:orc won't yet be the default in 1.6.

However, Araq says:

Here is my official nim.cfg recommendation:

...

--gc:orc # The one and only way to do memory management in modern Nim.

[ee7]

Configlet uses Nim 2.0 since https://github.com/exercism/configlet/commit/fa7d0bba04aa36a8b721a0e0b71d728bf3533d06, and --mm:orc is the default in Nim 2.0.

However, configlet explicitly uses --mm:refc for now: https://github.com/exercism/configlet/commit/d4e85bac4186518dd29d247df2e46fb2eb3d0ae3