Support non-contiguous edits (for deletions)

[ethan-leba]

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Currently the tree-edit syntax generation and rendering relies on the assumption that all modifications of the syntax tree would operate on a contiguous region (adjacent siblings).

For example, if we have ("(" identifier "," comment [identifier]) and perform a deletion, we'd like the result to be ("(" identifier comment), and the comment should be left unmodified.

Related to #18