Improve ci

[o0h]

• run tests with "lowest" ver. packages
  • check if the plugin run in lower ver(min:3.6+)
• run on PHP7.4snapshot
• generate code coverage using phpdbg
• phpstan lv.5
  • create phpstan.neon
  • addingTestCase dir to analyze target.

[codecov-io]

Codecov Report

Merging #19 into 2.x will increase coverage by 1.82%. The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x      #19      +/-   ##
============================================
+ Coverage     85.18%   87.01%   +1.82%     
+ Complexity       26       25       -1     
============================================
  Files             5        5              
  Lines            81       77       -4     
============================================
- Hits             69       67       -2     
+ Misses           12       10       -2

Impacted Files	Coverage Δ	Complexity Δ
src/Http/Client.php	90% <0%> (+3.63%)	11% <0%> (-1%)	:arrow_down:

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