Add UTF-16 for Position Encoding

[PalanixYT]

The LSP client needs to support UTF-16 for Position Encoding.
It is in spec for the server to ignore the PositionEncodingKind sent by the client and just use UTF-16 anyways.
This is the behaviour of Tinymist, which will break the position encodings.
For more information

[neurocyte]

This is not going to happen very soon. I know it's technically required by the standard, but very few LSPs actually don't support utf-8 positioning, and adding utf-16 positioning is likely a non-trivial change in flow.

In the mean time I suggest you check to see if perhaps a quick fix is possible inside tinymist.