Improve core log handling

[cmyr]

This adds a few things:

• it passes our logging directory to core as a launch argument (closes #298)
• it keeps the last 100 lines of core's stderr around, and dumps this to a file if core exits unexpectedly

It still doesn't feel like we handle the core connection very elegantly. It would be nice, for instance, if we showed an alert if core had crashed, maybe with an option to open the log directory. It would also be nice if we were better about how we managed the subprocess's shutdown, in general? We crash pretty hard currently.

[cmyr]

@mmatoszko @rakaramos @liamrosenfeld maybe one of you would like to give this a look?

[codecov-io]

Codecov Report

Merging #353 into master will decrease coverage by 1.74%. The diff coverage is 47.69%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
- Coverage   45.87%   44.12%   -1.75%     
==========================================
  Files          30       31       +1     
  Lines        3863     3884      +21     
==========================================
- Hits         1772     1714      -58     
- Misses       2091     2170      +79

Impacted Files	Coverage Δ
Tests/XiEditorTests/XiCoreTests.swift	94.62% <100%> (+0.3%)	:arrow_up:
Sources/XiEditor/RPCSending.swift	56.97% <43.33%> (-0.01%)	:arrow_down:
Sources/XiEditor/LoggingUtilities.swift	43.33% <43.33%> (ø)
Sources/XiEditor/EditViewController.swift	27.41% <0%> (-10.62%)	:arrow_down:
Sources/XiEditor/AppDelegate.swift	36.76% <0%> (-2.95%)	:arrow_down:
Sources/XiEditor/XiTextPlane/Atlas.swift	76.78% <0%> (-1.79%)	:arrow_down:
Sources/XiEditor/EditView.swift	54.44% <0%> (-0.86%)	:arrow_down:

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