fix: Calculate byte offset after applying each document change

[radeksimko]

Fixes #283

The diff is considerably large, but most of it is actually just code moving between packages and pointers being added to various places.

[appilon]

I am a little lost on this one, but everything looks good!

[radeksimko]

The root cause is partially explained in the conversation in the linked issue, but here is a summary:

When receiving didChange with a slice of >1 changes, we [previously] calculated the byte offset for all of them just after receiving the request, but that can only be done for the first one, because as per LSP spec, the positions are "relative" to the state of the document after previous changes (in the batch) were applied.

Hence I moved the byte offset calculation from lsp package (which shouldn't really do this kind of stuff anyway; it just felt appropriate because in order to convert "LSP position" into HCL, you need access to the full document to calculate the byte offset) into filesystem.

[ghost]

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