callr-based parallel runner

[richfitz]

Merge after #71, contains those commits

A parallel runner, powered by callr. Based on the same idea in mcstate

[codecov]

Codecov Report

Attention: Patch coverage is 91.26984% with 11 lines in your changes missing coverage. Please review.

Project coverage is 99.43%. Comparing base (3c0ffa2) to head (47ee434).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
R/runner-callr.R	90.90%	8 Missing ⚠️
R/samples.R	33.33%	2 Missing ⚠️
R/util.R	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   99.65%   99.43%   -0.23%     
==========================================
  Files          63       64       +1     
  Lines        4674     4796     +122     
==========================================
+ Hits         4658     4769     +111     
- Misses         16       27      +11     

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[richfitz]

The missing lines here are acceptable, and largely a product of the flakey codecov action - most are unrelated, and we can go through and mop them up later.

There are some missing lines here testing the progress bar bits. I'm not actually sure how best to do this, and might come up with some gory mocking later once we are happy with the behaviour in practice

[weshinsley]

This all runs nicely for me - Cascadia Mono is a good font to use, and somewhere in the docs we should mention that other fonts might not behave nicely.

Also codecov is complaining - covr::report() reports some things we could chase down in a future PR