Fix for 2272 - <f12> opens in current tab

[psxpaul]

This is a fix for #2272 to make language.gotoDefinition use the Oni.DefaultFileOpenOptions.openMode setting by default. It also adds:

• language.gotoDefinition.openNewTab
• language.gotoDefinition.openEdit
• language.gotoDefinition.openExistingTab

so that users can create bindings for the specific behavior they want.

[codecov]

Codecov Report

Merging #2504 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2504      +/-   ##
==========================================
+ Coverage    44.2%   44.22%   +0.01%     
==========================================
  Files         344      344              
  Lines       13886    13882       -4     
  Branches     1829     1828       -1     
==========================================
+ Hits         6138     6139       +1     
+ Misses       7508     7503       -5     
  Partials      240      240

Impacted Files	Coverage Δ
...er/src/Editor/NeovimEditor/NeovimEditorCommands.ts	12.28% <0%> (+0.95%)	⬆️
browser/src/Editor/NeovimEditor/Definition.ts	31.57% <0%> (+9.35%)	⬆️

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[akinsho]

@psxpaul sorry for the delay reviewing this, looks good to me 👍