Closing splits with 'editor.splits.mode' === 'oni'

[bryphe]

• Fix Window reference (OniEditor vs NeovimEditor)
• Fix commandline sticking around
• Dispose editor

[codecov]

Codecov Report

Merging #1703 into master will decrease coverage by 0.14%.
The diff coverage is 8%.

@@            Coverage Diff             @@
##           master    #1703      +/-   ##
==========================================
- Coverage    42.2%   42.06%   -0.15%     
==========================================
  Files         163      163              
  Lines        6205     6231      +26     
  Branches      870      878       +8     
==========================================
+ Hits         2619     2621       +2     
- Misses       3412     3436      +24     
  Partials      174      174

Impacted Files	Coverage Δ
browser/src/neovim/Session.ts	10% <0%> (-1.67%)	⬇️
browser/src/neovim/NeovimInstance.ts	5.79% <0%> (-0.13%)	⬇️
browser/src/Services/EditorManager.ts	50.79% <25%> (-2.78%)	⬇️
browser/src/UI/components/Tabs.tsx	39.58% <0%> (-0.85%)	⬇️
...src/Services/Configuration/DefaultConfiguration.ts	81.25% <0%> (ø)	⬆️
browser/src/Services/Workspace/Workspace.ts	23.15% <0%> (ø)	⬆️

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