Feature: Swift 6 preparation

[Brett-Best]

• Enabled upcoming/experimental swift feature flags
• Fixed most of the new warnings

Remaining are:

XCStringsTool
/Sources/StringExtractor/StringParser.swift
/Sources/StringExtractor/StringParser.swift:62:16 Static property 'regex' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

PluginTests
/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/plugins/xcstrings-tool.output/PluginTests/XCStringsToolPlugin/XCStringsTool/FeatureOne.swift
/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/plugins/xcstrings-tool.output/PluginTests/XCStringsToolPlugin/XCStringsTool/FeatureOne.swift:224:170 Static property 'featureOne' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/plugins/xcstrings-tool.output/PluginTests/XCStringsToolPlugin/XCStringsTool/Localizable.swift
/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/plugins/xcstrings-tool.output/PluginTests/XCStringsToolPlugin/XCStringsTool/Localizable.swift:260:171 Static property 'localizable' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

XCStringsToolTests
/Tests/XCStringsToolTests/GenerateTests.swift
/Tests/XCStringsToolTests/GenerateTests.swift:141:23 Static property 'sourceCode' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/checkouts/swift-snapshot-testing/Sources/SnapshotTesting/Snapshotting/String.swift
/DerivedData/xcstrings-tool-dmlhbkyuqhmcebaijwuzkgpsrtoy/SourcePackages/checkouts/swift-snapshot-testing/Sources/SnapshotTesting/Snapshotting/String.swift:11:21 Static property 'lines' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

[liamnichols]

I had to move the @MainActor annotations up to the class level in the tests. For some reason they were failing with the annotation on the functions.

[liamnichols]

Wait, that didn't work 👀 Maybe something related to pointfreeco/swift-snapshot-testing#822 (comment)?