Support lsp-1.5
This is still WIP, because I haven't figured out how to migrate parts of the code yet, e.g. https://github.com/haskell/lsp/issues/441.
BTW, I was wondering whether it's truly necessary or useful to maintain backwards compatibility with lsp and other libraries in dhall-lsp-server. After all, I strongly suspect that no-one is using this package as a library and would therefore have a need for backwards compatibility.
I highly recommend not trying to retain backwards compatibility with lsp. It's still moving relatively fast, and I anticipate releasing more breaking changes in the future. For HLS we pin to a particular version.