Add CI

[ghar1821]

First iteration of CI using R-lib actions to install dependencies and the package itself.

[tomashhurst]

@ghar1821 are these are failing because the branch does not yet have the full r-libs setup?

[ghar1821]

@tomashhurst More I don't think I set it up correctly. Those commits were pretty much trial and error to troubleshoot what caused the error. Github log wasn't clear what the error was, it only said

❯ checking whether package ‘Spectre’ can be installed ... [13s/14s] ERROR
  See below...

But then there was no indication as to what the error was, just a bunch of warnings. So then I tweaked the r-libs settings a bit and removed some of the manual test scripts in testthat folder (these are not automated unit tests). Only after then the error message popped out saying it has issues compiling the function documentations. Turns out, a comment in the example of run.pca was missing # which caused R CMD check to give out error when compiling examples.

So at the end, the numerous commits were my way of troubleshooting what has gone wrong. Once I managed to finalise the r-libs setting, I then did a test run and fix the errors locally before doing a commit.