Problem:

    vim.lsp.completion.enable(true, client.id, bufnr)
    vim.lsp.completion.enable(false, client.id, bufnr)

    Error detected while processing LspDetach Autocommands for "*":
    Error executing lua callback: …/lsp/completion.lua:701: Vim:E367: No such group: "vim/lsp/completion-22"
    stack traceback:
            [C]: in function 'nvim_del_augroup_by_name'
            …/lsp/completion.lua:701: in function 'disable_completions'
            …/lsp/completion.lua:724: in function 'enable'

Solution:
Delete the correct augroup.

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:
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`.

Deprecated:
- `client.request()` -> `client:request()`
- `client.request_sync()` -> `client:request_sync()`
- `client.notify()` -> `client:notify()`
- `client.cancel_request()` -> `client:cancel_request()`
- `client.stop()` -> `client:stop()`
- `client.is_stopped()` `client:is_stopped()`
- `client.supports_method()` -> `client:supports_method()`
- `client.on_attach()` -> `client:on_attach()`

Fixed docgen to link class fields to the full function doc.

* deprecate old signatures
* move to new str_byteindex/str_utfindex signature
* use single-underscore name (double-underscore is reserved for Lua itself)