[5.9] Improve Swift Macro template

[ahoppen]

Cherry-picks #6410 to release/5.9.

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• Use 4 spaces instead of 2 to be consistent with the other templates
• Use BasicMacroExpansionContext( instead of BasicMacroExpansionContext.init
• Add two newlines in the test method to improve readability
• Depend on the latest 509.0.0 prerelease of SwiftSyntax instead of main.

[ahoppen]

@swift-ci Please smoke test

[tomerd]

is this version what we want from a semantic versioning pov?

cc @neonichu

[ahoppen]

@neonichu and I discussed this and it’s what we want for now. As we approach the release, we want to change this to 509.0.0.