Editorconfig: specify indentation and dont overrule Makefile

[hansl]

Before, the Makefile rule was overridden as rules are evaluated in order and merged in order. Also, if your editor is set to a different tab size/style, it wasn't correct for this project.

This will avoid changes like these in PRs: https://github.com/boa-dev/boa/pull/4436/files#diff-5f57f7d9e7e1a6c330e90b09112acd05a90d11afde996c58dd056f69e30fc4c1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.37%. Comparing base (6ddc2b4) to head (168bff5).
⚠️ Report is 537 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4446      +/-   ##
==========================================
+ Coverage   47.24%   51.37%   +4.13%     
==========================================
  Files         476      504      +28     
  Lines       46892    51361    +4469     
==========================================
+ Hits        22154    26389    +4235     
- Misses      24738    24972     +234     

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