Follow up to https://github.com/neovim/neovim/pull/32072

If there is no prefix (e.g. at the start of word boundary or a line), it
always used the `filterText` because the `match` function always
returned false.

Problem:

With language servers like lemminx, completing xml tags like `<mo` first
shows the right candidates (`modules`) but after typing `d` the
candidates disappear.

This is because the server returns:

    [...]
    filterText = "<module",
    label = "module",
    textEdit = {
      newText = "<module>$1</module>$0",

Which resulted in `module` being used as `word`, and `module` doesn't
match the prefix `<mo`. Typing `d` causes the `complete()` filtering
mechanism to kick in and remove the entry.

Solution:

Use `<module` from the `filterText` as `word` if the textEdit/label
heuristic doesn't match.

Problem:  open_floating_preview() may be hidden behind current window if
          that is floating and has a higher zindex.
Solution: Open floating preview with zindex higher than current window.

Problem:
An LSP configuration that creates client with no root_dir or
workspace_folders can result in vim.lsp.enable attaching to it multiple
times.

Solution:
When checking existing clients, reuse a client if it wasn't initially
configured have any workspace_folders. This more closely matches the
behaviour we had prior to d9235ef

Not used anywhere.

Problem:
LSP module has multiple "start" interfaces.

Solution:
- Enhance vim.lsp.start
- Deprecate vim.lsp.start_client

* refactor(shared): extract `vim._list_insert` and `vim._list_remove`

* feat(lsp): add `vim.lsp.foldexpr()`

* docs(lsp): add a todo for state management

* feat(lsp): add `vim.lsp.folding_range.foldclose()`

* feat(lsp): schedule `foldclose()` if the buffer is not up-to-date

* feat(lsp): add `vim.lsp.foldtext()`

* feat(lsp): support multiple folding range providers

* refactor(lsp): expose all folding related functions under `vim.lsp.*`

* perf(lsp): add `lsp.MultiHandler` for do `foldupdate()` only once

Co-authored-by: Jesse <github@jessebakker.com>

Problem:
LSP spec uses the term "position encoding" where we say "offset encoding".

Solution:
- Rename it everywhere except `vim.lsp.Client.offset_encoding` (which would be breaking).
- Mention "position encoding" in the documentation for `vim.lsp.Client.offset_encoding`.

Problem: floaing preview window can be closed by some
ex commands like `only` `fclose` which will not clean the bufvar

Solution: use WinClosed event with floating_winnr for clean
bufnr, and add test cases for vim.lsp.util.open_floating_preview