negotiate position encoding (UTF-8, UTF-16, etc) with client and handle outcome

[jokeyrhyme]

• Rust strings are UTF-8, and JavaScript strings are UTF-16
• during initialisation, the client provides a list of compatible encodings, and the language server selects one and sends it back
• we've currently hard-coded UTF-8, and https://github.com/nushell/vscode-nushell-lang likely depends on the LSP default which is UTF-16
• we need to make sure that we support clients with either encoding, and that our text-traversing logic works in both cases

[jokeyrhyme]

in #13 , we hard-coded use of UTF-16 instead of UTF-8, but this is still hard-coding and not negotiating

[jokeyrhyme]

relates to: GiveMe-A-Name/lsp-textdocument#42