Unify identifier naming style with camelCase.

[youpong]

Description

Unify identifier naming style with camelCase.

• Path::is_executable() -> Path::isExecutable()
• Path::is_directory() -> Path::isDirectory()
• Path::is_regular_file() -> Path::isRegularFile()
• Path::to_s() -> Path::toString()

[codecov-commenter]

Codecov Report

Merging #86 (446d351) into main (775295b) will not change coverage. The diff coverage is 57.14%.

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   27.36%   27.36%           
=======================================
  Files           4        4           
  Lines         391      391           
=======================================
  Hits          107      107           
  Misses        284      284           

Impacted Files	Coverage Δ
src/fetch.cpp	0.00% <0.00%> (ø)
src/fetch.h	80.88% <100.00%> (ø)
src/util.cpp	100.00% <100.00%> (ø)

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