Improve message types for `rix_init()` and `with_nix()`

[philipp-baumann]

• [x] rix_init(): add options c("simple", "quiet"). Default to simple.

• [x] combine two sentences for with_nix(): is_nix_rsession(), is_rstudio_session()

• [x] remove entries in with_nix() for option "simple":

==> Process ID (PID) is 57429.
==> Receiving stdout and stderr streams...

• [x] robustify message_type args everywhere in nix_init() and with_nix() and their internal helpers.

• [x] Only keep session info for verbose option.

[philipp-baumann]

addressed with:

• https://github.com/b-rodrigues/rix/commit/2f1dfdae37abb1387cc68862241c79c3bc5df3f5
• https://github.com/b-rodrigues/rix/commit/9b816b873b7ff1dba1fc21a89bfa563331cf956d
• https://github.com/b-rodrigues/rix/commit/5d86bb5e18f592f65276627b1ddd90ce332c1ed4
• https://github.com/b-rodrigues/rix/commit/925f815f6cf91020440cb975a66f3992656849ef
• https://github.com/b-rodrigues/rix/commit/0bf1b8edf5cfdb30a8e3f350be3dfa35f377287e
• https://github.com/b-rodrigues/rix/commit/12efbef04b8d3a6b64c3801d2718399e23f328e9
• https://github.com/b-rodrigues/rix/commit/608a656fc3bbe8f2642e232301f77067e425feb6
• https://github.com/b-rodrigues/rix/commit/e47ba571649e384bf91c8bf57f4b259b8a3091ee
• https://github.com/b-rodrigues/rix/commit/a207a08f23b753870d6984f5e17fa93ac8c7aa69
• https://github.com/b-rodrigues/rix/commit/fa6b9f295ab86ccead414fe3bf4bee31ad45facf : along with improving internal docs

[philipp-baumann]

finalized with #180