`BasicFormat` doesn't add spaces between first and second parameter name

[stevapple]

Description

When using FunctionParameterSyntax with SwiftSyntaxBuilder, the build result (formatted by BasicFormat) doesn't have spaces between first and second parameter names, which is "lexically legal" but far against user intention.

Steps to Reproduce

let parameter = FunctionParameterSyntax(firstName: "on", secondName: "eventLoop", colon: .colonToken(), type: TypeSyntax("EventLoop"))
print(parameter) // oneventLoop: EventLoop

[ahoppen]

rdar://107787208