nvim: add SetBufferText and Echo apis

[zchee]

Add SetBufferText and Echo apis.

> nvim_buf_set_text(buffer Buffer, start_row Integer, start_col Integer, end_row Integer, end_col Integer, replacement ArrayOf(String)) void { name(nvim_buf_set_text) } > nvim_echo(chunks Array, history Boolean, opts Dictionary) void { name(nvim_echo) } 

[codecov]

Codecov Report

Merging #96 (e196786) into master (d643d77) will increase coverage by 0.5%.
The diff coverage is 50.0%.

@@ Coverage Diff @@ ## master #96 +/- ## ======================================== + Coverage 62.8% 63.3% +0.5%  ======================================== Files 14 14 Lines 2081 2085 +4 ======================================== + Hits 1308 1321 +13  + Misses 636 627 -9  Partials 137 137 

Flag	Coverage Δ
linux-1.13	63.3% <50.0%> (+0.5%)	⬆️
linux-1.14	63.3% <50.0%> (+0.5%)	⬆️
linux-1.15	63.3% <50.0%> (+0.5%)	⬆️
linux-1.16	63.3% <50.0%> (+0.5%)	⬆️
macos-1.13	63.3% <50.0%> (+0.5%)	⬆️
macos-1.14	63.3% <50.0%> (+0.5%)	⬆️
macos-1.15	63.3% <50.0%> (+0.5%)	⬆️
macos-1.16	63.3% <50.0%> (+0.5%)	⬆️
windows-1.13	62.5% <50.0%> (+0.5%)	⬆️
windows-1.14	62.5% <50.0%> (+0.5%)	⬆️
windows-1.15	62.5% <50.0%> (+0.5%)	⬆️
windows-1.16	62.5% <50.0%> (+0.5%)	⬆️

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Impacted Files	Coverage Δ
nvim/types.go	0.0% <ø> (ø)
nvim/api.go	38.1% <50.0%> (+3.0%)	⬆️

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