Document environment variables that can be used to modify swift-syntax’s build or test execution

rdar://112473317
Fixes #1922

Author: ahoppen

Contributor Documentation/Environment variables.md

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+# Environment variables
+
+The following environment variables can be used to control some behavior in swift-syntax.
+
+## Build time
+
+- `SWIFT_BUILD_SCRIPT_ENVIRONMENT`: Enable assertions in release builds and remove the dependency of swift-syntax on os_log.
+- `SWIFTCI_USE_LOCAL_DEPS`: Assume that all of SourceKit-LSP’s dependencies are checked out next to it and use those instead of cloning the repositories. Primarily intended for CI environments that check out related branches.
+- `SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION`: Mutate the input of `assertParse` test cases. See [CONTRIBUTING](../CONTRIBUTING.md) for more information.
+- `SWIFTSYNTAX_BUILD_DYNAMIC_LIBRARIES`: Build all libraries as *dynamic* instead of *automatic*. This allows us to build swift-syntax as dynamic libraries, which in turn allows us to build SourceKit-LSP using SwiftPM on Windows. Linking swift-syntax statically into sourcekit-lsp exceeds the maximum number of exported symbols on Windows.
+- `SWIFTSYNTAX_ENABLE_RAWSYNTAX_VALIDATION`: Check that the layout of the syntax tree is correct when creating or modifying nodes. See [CONTRIBUTING](../CONTRIBUTING.md) for more information.
+
+## Testing
+
+- `SKIP_LONG_TESTS`: Skip tests that typically take more than 1s to execute.