Store top-level whitespace in the AST

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The aid full-length file serialization, the AST could contain the information about the whitespace. For starters, the parser could yield the top-level whitespace between other entries.

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I'm having seconds thoughts on this one. What can top-level whitespace do that calculating spans between entries (in tooling parsers) can't?

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As per the OTP triage, I moved this out of the Syntax 0.7 scope into FUTURE. There aren't strong drivers for this feature right now. At Mozilla, compare-locales and cross-channel might benefit from it, but they use their own span-based solution right now (here and here).

Let's consider this in the future, when the issue of storing all whitespace is discussed (#121). We might want to employ a common solution to both the top-level and the in-entry whitespace.