Remove get virtual text fix extmark

[zchee]

Remove get_virtual_text api and fix extmark raleted args

[h-michael]

@zchee It seems that we should update codecov GHA. ref: #79

[zchee]

@h-michael sorry for late.
Your PR might be works codecov! but seems to atill draft, Can I just copy code to this PRfrom your PR, and close #79?

[codecov]

Codecov Report

Merging #78 (7a87a37) into master (acbae1a) will increase coverage by 1.3%.
The diff coverage is 50.0%.

@@           Coverage Diff            @@
##           master     #78     +/-   ##
========================================
+ Coverage    56.1%   57.4%   +1.3%     
========================================
  Files          11      11             
  Lines        2636    2124    -512     
========================================
- Hits         1479    1221    -258     
+ Misses       1021     769    -252     
+ Partials      136     134      -2     

Impacted Files	Coverage Δ
nvim/apiimp.go	24.1% <50.0%> (+2.1%)	⬆️
nvim/plugin/plugin.go	31.5% <0.0%> (-3.9%)	⬇️
msgpack/encode.go	76.7% <0.0%> (-3.6%)	⬇️
msgpack/rpc/endpoint.go	58.0% <0.0%> (-3.2%)	⬇️
msgpack/decode.go	63.7% <0.0%> (-3.1%)	⬇️
msgpack/unpack.go	82.9% <0.0%> (-2.3%)	⬇️
msgpack/common.go	70.7% <0.0%> (-2.0%)	⬇️
nvim/plugin/main.go	31.1% <0.0%> (-0.2%)	⬇️
msgpack/pack.go	87.5% <0.0%> (-0.1%)	⬇️
nvim/nvim.go	68.8% <0.0%> (+0.1%)	⬆️
... and 2 more

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[zchee]

@h-michael PTAL.