[Macros] Attached macro expansions return single string
* Move collapse(expansions:for:attachedTo:) to SwiftSyntaxMacroExpansion
* SwiftSyntaxMacroExpansion.expandAttachedMacro() now perform collapsing
* SwiftSyntaxMacroExpansion.expandAttachedMacroWithoutCollapsing()
to keep old behavior
* IPC request 'getCapability' now sends the host protocol version
* Unified IPC response 'macroExpansionResult' that returns single string
for both 'expandFreestandingMacro' and 'expandAttachedMacro'
* Compiler accepts old 'expandFreestandingMacroResult' and
'expandAttachedMacroResult' to keep compatibility
* Plugins check the compiler's protcol version to see if it suppports
'macroExpansionResult', and fall back to old behavior if necessary
