Support retrieving comments inserted above keys

[mattolenik]

Given the following:

// This is some description
// of the object below.
data "machine" {
    foo = 123
}

Users should be able to query something like:

hclq get-comments data.machine

And the value This is some description of the object below. would be returned.

I think this may prove useful for using comments as "annotations." This could be used to add additional type data, linting rules, other requirements, whatever you want.