nvim: add skipVersion test util for API version compatibility

[zchee]

nvim: add skipVersion test util for API version compatibility.

[codecov]

Codecov Report

Merging #119 (35e18ab) into master (c8dbb8f) will increase coverage by 0.4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #119     +/-   ##
========================================
+ Coverage    63.9%   64.4%   +0.4%     
========================================
  Files          14      14             
  Lines        2114    2114             
========================================
+ Hits         1352    1362     +10     
+ Misses        626     615     -11     
- Partials      136     137      +1     

Flag	Coverage Δ
linux-1.15	64.4% <ø> (+0.4%)	⬆️
linux-1.16	64.4% <ø> (+0.4%)	⬆️
linux-1.17	64.4% <ø> (+0.4%)	⬆️
macos-1.15	64.3% <ø> (+0.3%)	⬆️
macos-1.16	64.4% <ø> (+0.4%)	⬆️
macos-1.17	64.4% <ø> (+0.5%)	⬆️
windows-1.15	63.6% <ø> (+0.4%)	⬆️
windows-1.16	63.6% <ø> (+0.4%)	⬆️
windows-1.17	63.6% <ø> (+0.4%)	⬆️

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Impacted Files	Coverage Δ
nvim/api.go	40.7% <0.0%> (+0.5%)	⬆️
msgpack/decode.go	76.5% <0.0%> (+2.2%)	⬆️

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