Externalized UI: Implement Oni split mode

[bryphe]

This implements basic split rendering for Oni (this isn't on by default, is only available with oni.splits.mode: 'oni', and there isn't an entry point to hit this other than calling directly in the console)

But it lets you do crazy stuff like this:

I don't even know what is going on there 😄

[bryphe]

• Finish wiring up commands for splits from window motions
• Fix bug with split direction (44c1e4e)
• Fix bottom margin

[codecov]

Codecov Report

Merging #1682 into master will increase coverage by 0.47%.
The diff coverage is 84.44%.

@@            Coverage Diff             @@
##           master    #1682      +/-   ##
==========================================
+ Coverage   46.79%   47.26%   +0.47%     
==========================================
  Files         121      122       +1     
  Lines        4505     4540      +35     
  Branches      649      656       +7     
==========================================
+ Hits         2108     2146      +38     
+ Misses       2244     2242       -2     
+ Partials      153      152       -1

Impacted Files	Coverage Δ
...r/src/Services/WindowManager/WindowManagerStore.ts	50% <ø> (ø)	⬆️
...rowser/src/Services/WindowManager/WindowManager.ts	20.45% <0%> (-1.5%)	⬇️
.../src/Services/WindowManager/layoutFromSplitInfo.ts	100% <100%> (ø)
.../src/Services/WindowManager/LinearSplitProvider.ts	61.33% <100%> (+17.39%)	⬆️
browser/src/Services/WindowManager/index.ts	70.58% <100%> (+1.83%)	⬆️
browser/src/UI/components/Sneakable.tsx	26.31% <0%> (+4.09%)	⬆️
browser/src/Services/Sneak/index.tsx	41.66% <0%> (+5.3%)	⬆️
.../src/Services/WindowManager/SingleSplitProvider.ts	88.88% <0%> (+11.11%)	⬆️

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[bryphe]

I'll bring this in for now, but keeping the configuration setting off (editor.split.mode needs to be set to 'oni' to enable this functionality). There are still lots of bugs / issues to be fixed before this becomes useful (like you can't close a split right now without #1703), and we don't properly sync the shada state between nvim instances, which means stuff like marks and yanks don't get persisted between splits, which would be very confusing.