- not necessary on master: got lost in the vim.lsp.diagnostic => vim.diagnostic migration
- fix tests which accidentally depended on previous session
- ref #15504

According to the protocol definition `rootPath`, `rootUri` and
`workspaceFolders` are allowed to be null.

Some language servers utilize this to provide "single file" support.
If all three are null, they don't attempt to index a directory but
instead only provide capabilities for a single file.

Some language servers *cough*rust-analyzer*cough* need an empty/custom
workspaceFolders for certain usecases. For example, rust-analyzer
needs an empty workspaceFolders table for standalone file support
(See https://github.com/rust-analyzer/rust-analyzer/pull/8955).

This can also be useful for other languages that need to commonly
open a certain directory (like flutter or lua), which would help
prevent spinning up a new language server altogether.

In case no workspaceFolders are passed, we fallback to what we had
before.

ipairs terminates on the first nil index when iterating over table keys:

for i,k in ipairs( {[1] = 'test', [3] = 'test'} ) do
  print(i, k)
end

prints:
1 test

Instead, use pairs which continues iterating over the entire table:

for i,k in pairs( {[1] = 'test', [3] = 'test'} ) do
  print(i, k)
end

prints:
1 test
3 test

- Fix the cursor position after applying TextEdits
- Support reversed range of TextEdit
- Invoke nvim_buf_set_text one by one

* Simplify rpc encode/decode messages to rpc.send/rcp.receive
* Make missing handlers message throw a warning
* Clean up formatting style in log
* Move all non-RPC loop messages to trace instead of debug
* Add format func option to log to allow newlines in per log entry

This fixes the handler signature and also prevents n+1 requests firing
if there are multiple clients.

(The first `prepareCallHierarchy` handler is called once per client,
each invocation used `buf_request` to make more requests using *all*
clients)

Follow up to https://github.com/neovim/neovim/pull/15504