Pass along cancellation token in snippets

[CyrusNajmabadi]

Thread through a cancellation token from a callee parameter instead of silently ignoring it.

Note that this will not have any product impact on cancellation in snippets, since a caller further up the chain still hard-codes CancellationToken.None.

roslyn/src/VisualStudio/Core/Def/Snippets/SnippetFunctions/AbstractSnippetFunction.IVsExpansionFunction.cs

Lines 15 to 19 in 1401f9b

	int IVsExpansionFunction.GetDefaultValue(out string bstrValue, [ComAliasName("Microsoft.VisualStudio.OLE.Interop.BOOL")] out int fHasDefaultValue)
	=> GetDefaultValue(CancellationToken.None, out bstrValue, out fHasDefaultValue);
	int IVsExpansionFunction.GetCurrentValue(out string bstrValue, out int fHasCurrentValue)
	=> GetCurrentValue(CancellationToken.None, out bstrValue, out fHasCurrentValue);

[CyrusNajmabadi]

@akhera99 @sharwell ptal.

[sharwell]

Approving as cleaner code, but edited the initial comment to call out that this won't have any impact on runtime scenarios without additional changes.