Support .editorconfig

[timsneath]

When formatting a file, we should respect settings in .editorconfig such as tab width and maximum line length.

More details on the editorconfig format: https://editorconfig.org

Editor config settings help a project retain consistency, and are increasingly pervasive. Both VSCode and Xcode 16+ now support this file format.

[ahoppen]

Synced to Apple’s issue tracker as rdar://132749844

[allevato]

A couple questions we'd need good answers for:

• If the .editorconfig file has settings that conflict with the swift-format configuration, which take precedence? It looks like Prettier supports .editorconfig files and the Prettier config overrides .editorconfig, which feels like the right answer—it's the more specific one.
• The search semantics of .editorconfig are a little different; the effective configuration in a directory is the merging of all .editorconfig files from that directory upward until a root configuration is reached. swift-format has similar tree-searching logic except we stop upon finding a configuration file and ignore parents above that. So we'd have to implement slightly different behavior when reading those to support the spec properly.

[bnbarham]

config overrides .editorconfig, which feels like the right answer—it's the more specific one.

Yep, I'd agree with this too.

[shawnhyam]

When naming an explicit configuration file, we skip the normal logic for traversing the directories to find a .swift-format file. In this case, do you think we would still want to follow the normal logic for finding .editorconfig files, or skip it as well?