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[bot]]

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%)	:arrow_up:
linux-1.14	63.3% <50.0%> (+0.5%)	:arrow_up:
linux-1.15	63.3% <50.0%> (+0.5%)	:arrow_up:
linux-1.16	63.3% <50.0%> (+0.5%)	:arrow_up:
macos-1.13	63.3% <50.0%> (+0.5%)	:arrow_up:
macos-1.14	63.3% <50.0%> (+0.5%)	:arrow_up:
macos-1.15	63.3% <50.0%> (+0.5%)	:arrow_up:
macos-1.16	63.3% <50.0%> (+0.5%)	:arrow_up:
windows-1.13	62.5% <50.0%> (+0.5%)	:arrow_up:
windows-1.14	62.5% <50.0%> (+0.5%)	:arrow_up:
windows-1.15	62.5% <50.0%> (+0.5%)	:arrow_up:
windows-1.16	62.5% <50.0%> (+0.5%)	:arrow_up:

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

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