Add `.editorconfig` file #309

Job	Run time
Swift 6 Language Mode / Swift 6 language mode	1m 7s
Integration Tests / Linux (5.8)	12m 54s
Integration Tests / Linux (5.9)	14m 9s
Soundness / API breakage check	2m 36s
Integration Tests / Linux (5.10)	14m 25s
Soundness / Documentation check	2m 3s
Soundness / Broken symlinks check	5s
Integration Tests / Linux (nightly-6.0)	17m 10s
Soundness / Format check	1m 1s
Integration Tests / Linux (nightly-main)	17m 37s
Soundness / License headers check	8s
Soundness / Shell check	6s
Soundness / Unacceptable language check	4s
Benchmarks / Benchmarks / Linux (5.8)	2m 38s
Benchmarks / Benchmarks / Linux (5.9)	3m 47s
Benchmarks / Benchmarks / Linux (5.10)	3m 40s
Benchmarks / Benchmarks / Linux (nightly-6.0)	4m 43s
Benchmarks / Benchmarks / Linux (nightly-main)	5m 16s
Cxx interop / Cxx interop / Linux (5.8)	1m 2s
Unit tests / Unit tests / Linux (5.8)	3m 19s
Cxx interop / Cxx interop / Linux (5.9)	1m 9s
Unit tests / Unit tests / Linux (5.9)	3m 25s
Cxx interop / Cxx interop / Linux (5.10)	1m 14s
Unit tests / Unit tests / Linux (5.10)	3m 39s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 48s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 51s
Unit tests / Unit tests / Linux (nightly-6.0)	4m 23s
Unit tests / Unit tests / Linux (nightly-main)	4m 19s
	2h 9m 38s

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