Problem:
`lsp.buf_request` send the same params to all servers and many
calls to this pass PositionalParams which depends on the clients
offset_encoding. This can result with incorrect params being sent
to a server.
Solution:
`lsp.buf_request` `params` argument can now be passed as a function
which takes the client as the first argument. This is used in
lsp/buf.lua to construct correct params for each client request.
2dcbfe78fc
