feat: Support reading config from non-regular files

[patrick96]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Other: Replace this with a description of the type of this PR

Description

We had a check that restricted config files to "regular files".
This check was to restrictive as it didn't allow for things like:

polybar -c <(gen_config)
gen_config | polybar -c /dev/stdin

Now, polybar can easily read config data from stdin.

Related Issues & Documents

Documentation (check all applicable)

• This PR requires changes to the Wiki documentation (describe the changes)
• This PR requires changes to the documentation inside the git repo (please add them to the PR).
• Does not require documentation changes

[codecov]

Codecov Report

Merging #2545 (caf4e79) into master (b5fb442) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2545      +/-   ##
==========================================
+ Coverage   10.82%   10.85%   +0.03%     
==========================================
  Files         150      150              
  Lines       10746    10759      +13     
==========================================
+ Hits         1163     1168       +5     
- Misses       9583     9591       +8     

Flag	Coverage Δ
unittests	10.85% <0.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
include/utils/file.hpp	77.77% <ø> (ø)
src/components/config_parser.cpp	44.51% <0.00%> (-0.28%)	⬇️
src/utils/file.cpp	31.60% <0.00%> (+0.74%)	⬆️

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