refactor function joinPaths and isAscii in utils.go

[suwakei]

I would suggest some improvements to further improve robustness and readability

##Proposal

Improve robustness of joinPaths function

Currently, the joinPaths function uses a homebrew lastChar function to determine if a path ends with a trailing slash. lastChar is a potential risk because it panics when passed an empty string. Replacing this logic with strings.HasSuffix, which is safer and clearer in intent, improves robustness and readability.
This change also eliminates the need for the lastChar function,
making the code base simpler.

Simplify isASCII functions

The current isASCII function is implemented using a for loop; it can be rewritten to be more modern and concise using strings.ContainsFunc, available in Go 1.19 and later. is safe.

Delete unnecessary tests

In conjunction with the removal of the lastChar function, we have also removed the associated test in utils_test.go.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.92%. Comparing base (3dc1cd6) to head (e8622bf).
Report is 142 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4288      +/-   ##
==========================================
- Coverage   99.21%   98.92%   -0.30%     
==========================================
  Files          42       44       +2     
  Lines        3182     3431     +249     
==========================================
+ Hits         3157     3394     +237     
- Misses         17       26       +9     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.85% <100.00%> (?)
-tags go_json	98.85% <100.00%> (?)
-tags nomsgpack	98.90% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	98.92% <100.00%> (?)
go-1.24	98.92% <100.00%> (?)
macos-latest	98.92% <100.00%> (-0.30%)	⬇️
ubuntu-latest	98.92% <100.00%> (-0.30%)	⬇️

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[suwakei]

Codecov's overall project coverage is showing as -0.30% and failed, but for my changes (diffs), everything is covered by the tests (100%).
Please contact me to discuss additional tests if needed.

[suwakei]

Hi! Just following up to see if there's anything I can improve in this PR 🙇

[suwakei]

Hi maintainers 👋
Just following up on this PR — happy to make any changes if needed. Please let me know if there's anything blocking review. Thanks!