[3.x] Add gitattributes

[viezel]

Improve the distribution size of this package, as there is no reason to ship tests, github workflow and various test files with composer.

[codecov]

Codecov Report

Merging #392 into master will decrease coverage by 0.30%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #392      +/-   ##
============================================
- Coverage     88.94%   88.64%   -0.31%     
  Complexity      469      469              
============================================
  Files            65       65              
  Lines          1303     1303              
============================================
- Hits           1159     1155       -4     
- Misses          144      148       +4     

Impacted Files	Coverage Δ	Complexity Δ
src/Commands/Install.php	88.23% <0.00%> (-11.77%)	6.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c2c90ff...112bb10. Read the comment docs.

[viezel]

@stancl no idea why it thinks code coverage went down, as I did not add any executable code in this PR. 🤷🏻‍♂️

[stancl]

Makes sense, good idea!