Add a basic .editorconfig

[aaronraimist]

https://editorconfig.org

Basically just makes it so I don't have to change my editor's indentation settings for every file.

[codecov-io]

Codecov Report

Merging #4257 into develop will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4257      +/-   ##
===========================================
- Coverage    73.55%   73.54%   -0.01%     
===========================================
  Files          299      299              
  Lines        29806    29806              
  Branches      4872     4872              
===========================================
- Hits         21923    21922       -1     
+ Misses        6456     6454       -2     
- Partials      1427     1430       +3

Impacted Files	Coverage Δ
synapse/handlers/user_directory.py	70.73% <0%> (-0.31%)	⬇️
synapse/handlers/search.py	81.25% <0%> (ø)	⬆️
synapse/handlers/federation.py	61.72% <0%> (ø)	⬆️

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[richvdh]

I'll take your word on it.