The idea here is to unify the different ways in which we can currently set options on SourceKit-LSP in a scalable way: Environment variables, command line arguments to `sourcekit-lsp` and initialization options.
The idea is that a user can define a `~/.sourcekit-lsp/.sourcekit-lsp` file (we store logs in `~/.sourcekit-lsp/logs` on non-Darwin platforms), which will be used as the default configuration for all SourceKit-LSP instances. They can also place a `.sourcekit-lsp` file in the root of a workspace to configure SourceKit-LSP for that project specifically, eg. setting arguments that need to be passed to `swift build` for that project and which thus also need to be set on SourceKit-LSP.
For compatibility reasons, I’m mapping the existing command line options into the new options structure for now. I hope to delete the command line arguments in the future and solely rely on `.sourcekit-lsp` configuration files.
Environment variable will be migrated to `.sourcekit-lsp` in a follow-up commit.
# Conflicts:
# Sources/SourceKitLSP/SourceKitLSPServer+Options.swift
# Sources/SourceKitLSP/Swift/SwiftLanguageService.swift
# Sources/sourcekit-lsp/SourceKitLSP.swift
# Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
# Tests/SourceKitLSPTests/ExecuteCommandTests.swift
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