feat(wdl-lint): support doc comments in MetaSections

[Serial-ATA]

part of #557

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[a-frantz]

I think this should be consistent instead of decided ad hoc for each node type. I'm not sure which is preferable (always include "floating" doc comments or never), but I think we should pick one and apply that everywhere.

[Serial-ATA]

floating comments are only allowed for preambles, which I think makes sense:

## This is a preamble

version 1.3

## This is a comment for `foo`
task foo {}

In that example, the comment on foo is clearly attached to the task definition. The preamble, though, is logically attached to the document and not the version statement, despite us internally treating the version statement as the documented node.

[a-frantz]

OOS for this PR, but should we start linting <1.2 structs for doc comment presence? They can't have meta sections within spec, but no reason they can't have doc comments.

[Serial-ATA]

sure, I'll make an issue after this is merged