nvim: support neovim/neovim@561df30 APIs

[zchee]

Support neovim/neovim@561df30 APIs.

> nvim_add_user_command(name String, command Object, opts Dictionary) void { name(nvim_add_user_command) }
> nvim_buf_add_user_command(buffer Buffer, name String, command Object, opts Dictionary) void { name(nvim_buf_add_user_command) }
> nvim_buf_del_user_command(buffer Buffer, name String) void { name(nvim_buf_del_user_command) }
> nvim_del_user_command(name String) void { name(nvim_del_user_command) }
> nvim_get_option_value(name String, opts Dictionary) Object { name(nvim_get_option_value) }
> nvim_set_option_value(name String, value Object, opts Dictionary) void { name(nvim_set_option_value) }

[codecov]

Codecov Report

Merging #126 (37f6413) into master (db8d22d) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #126     +/-   ##
========================================
+ Coverage    80.5%   80.6%   +0.1%     
========================================
  Files          14      14             
  Lines        3018    3044     +26     
========================================
+ Hits         2430    2456     +26     
  Misses        588     588             

Flag	Coverage Δ
linux-1.15-nightly	80.6% <100.0%> (+0.1%)	⬆️
linux-1.15-stable	79.8% <7.6%> (-0.7%)	⬇️
linux-1.16-nightly	80.6% <100.0%> (+0.1%)	⬆️
linux-1.16-stable	79.8% <7.6%> (-0.7%)	⬇️
linux-1.17-nightly	80.6% <100.0%> (+0.1%)	⬆️
linux-1.17-stable	79.8% <7.6%> (-0.7%)	⬇️
macos-1.15-nightly	80.6% <100.0%> (+0.1%)	⬆️
macos-1.15-stable	79.8% <7.6%> (-0.7%)	⬇️
macos-1.16-nightly	80.6% <100.0%> (+0.1%)	⬆️
macos-1.16-stable	79.8% <7.6%> (-0.7%)	⬇️
macos-1.17-nightly	80.6% <100.0%> (+0.1%)	⬆️
macos-1.17-stable	79.8% <7.6%> (-0.7%)	⬇️
windows-1.15-nightly	79.8% <100.0%> (+0.1%)	⬆️
windows-1.15-stable	79.0% <7.6%> (-0.7%)	⬇️
windows-1.16-nightly	79.8% <100.0%> (+0.1%)	⬆️
windows-1.16-stable	79.0% <7.6%> (-0.7%)	⬇️
windows-1.17-nightly	79.8% <100.0%> (+0.1%)	⬆️
windows-1.17-stable	79.0% <7.6%> (-0.7%)	⬇️

Impacted Files	Coverage Δ
nvim/api.go	96.9% <100.0%> (+0.1%)	⬆️
nvim/types.go	88.8% <100.0%> (+1.3%)	⬆️

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