Add fsext.Abs helper function

[oleiade]

What?

This PR adds a fsext.Abs helper function to facilitate computing the absolute path of a file.

Why?

Similar code already exists in the current master branch:

k6/js/initcontext.go

Line 23 in 5e83d38

// Here IsAbs should be enough but unfortunately it doesn't handle absolute paths starting from

and as the fs module also relies on the same operation, I took this opportunity to refactor it in a the lib/fsext package.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

ref #3165 (blocks)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0db029a) 73.34% compared to head (6fb6f99) 73.38%.
Report is 2 commits behind head on master.

❗ Current head 6fb6f99 differs from pull request most recent head 3a16a29. Consider uploading reports for the commit 3a16a29 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3452      +/-   ##
==========================================
+ Coverage   73.34%   73.38%   +0.03%     
==========================================
  Files         261      261              
  Lines       19769    19779      +10     
==========================================
+ Hits        14500    14514      +14     
+ Misses       4371     4368       -3     
+ Partials      898      897       -1     

Flag	Coverage Δ
ubuntu	73.32% <94.73%> (+0.05%)	⬆️
windows	73.22% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cmd/root.go	94.37% <100.00%> (+0.14%)	⬆️
cmd/version.go	100.00% <100.00%> (ø)
js/initcontext.go	93.54% <100.00%> (-0.90%)	⬇️
lib/fsext/filepath.go	100.00% <100.00%> (ø)
lib/strvals/parser.go	100.00% <100.00%> (ø)
log/file.go	79.66% <100.00%> (ø)
log/loki.go	69.08% <100.00%> (ø)

... and 3 files with indirect coverage changes

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