Write internal text handling routines

[PMunch]

The LSP specification says that all code will be transported over UTF-8 in a JSON object. However it sends position as the UTF-16 representation. Yes this is dumb, but while it stays this way we must conform to it. So if anyone could do an internal text representation module that supports the TextEdit specification from the spec it would be really helpful for when we can start implementing the actual functionality.

[PMunch]

This was more or less done in https://github.com/PMunch/nimlsp/blob/master/src/nimlsppkg/utfmapping.nim. It doesn't support updating the finger table based on incremental changes, so currently the entire file is requested for the didChange requests.