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.

When `root_dir` is a function it can (and often will) call the provided
callback function in a fast API context (e.g. in the `on_exit` handler
of `vim.system`). When the callback function is executed we should
ensure that it runs vim.lsp.start on the main event loop.

Problem:
The floating window for hover and signature help always cuts off a few lines,
because the `_make_floating_popup_size` function counts empty lines as having
zero height.

Solution:
Ensure the height is at least 1.

Problem: health can not shown in a floating window
Solution: add g:health variable

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.

Allows to retrieve the configuration as it will be used by `lsp.enable`
- including the parts merged from `*` and rtp.

This is useful for explicit startup control
(`vim.lsp.start(vim.lsp.config[name])`)

Closes https://github.com/neovim/neovim/issues/31640

If root_dir is a function it is evaluated when the client is created to
determine the root directory.

This enables dynamically determining the root directory based on e.g.
project or directory structure (example: finding a parent Cargo.toml
file that contains "[workspace]" in a Rust project).

- Replace all uses of vim.regex with simpler Lua patterns.
- Replace all uses of vim.fn.substitute with string.gsub.
- Rework error handling so expected errors are passed back via a return.
  - These get routed up an passed to `vim.notify()`
  - Any other errors will cause a stack trace.
- Reworked the module initialization of `localfile_arg`
- Updated all type annotations.
- Refactored CLI completion by introduction a parse_cmdline()
  function.
- Simplified `show_toc()`
- Refactor highlighting
- Inline some functions
- Fix completion on MacOS 13 and earlier.
  - Prefer `manpath -q` over `man -w`
- Make completion more efficient by avoiding vim.fn.sort and vim.fn.uniq
  - Reimplement using a single loop

Problem:
regression since https://github.com/neovim/neovim/pull/31340

`nvim -l repro.lua`:
```lua
vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls' }
vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls', root_dir = 'foo' }

-- swapped case will be ok:
-- vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls', root_dir = 'foo' }
-- vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls' }
```

Failure:
```
E5113: Error while calling lua chunk: /…/lua/vim/lsp.lua:214: bad argument #1 to
'ipairs' (table expected, got nil)
stack traceback:
        [C]: in function 'ipairs'
        /…/lua/vim/lsp.lua:214: in function 'reuse_client'
        /…/lua/vim/lsp.lua:629: in function 'start'
        repro.lua:34: in main chunk
```