This is a bit more boilerplate because of painful impedance mismatch (is
augroup names the root source of truth? is ids the source of truth? who
knows? what does truth even mean?), but show an error reflecting what
the caller actually tried (they passed in an invalid id, not a null
string)

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

Problem:
Options parsing is still painful for dict-style options.

Solution:
schema-maxxing => better `opt:get()` (will be the basis for `vim.o()`),
unified (and more-detailed) err msgs.

- Drop bespoke structure-builder in `_core/options.lua`.
- Define `schema` for all non-primitive options (except 'guicursor' and
  statusline-style options); generate reified keysets `OptKeyDict`).
  - Generate 'fillchars' => `fcs_tab`, 'listchars' => `lcs_tab`.
- `nvim_set_option_value`:
  - Return the improved structures. Also from `vim.opt.x:get()`.
  - Eliminate api <=> lua roundtrip, centralize option structure
    handling.
- Improve/unify errors.
  - Bump ERR_BUFLEN 80 → 256 so the "one of" list isn't truncated.
- Eliminate old 'diffopt' order-dependence (`iwhiteall` before `iwhite`)

Error samples:

    Typed-key path (opt_strings_check → diffopt/mousescroll/breakindentopt):
    E474: Unknown item 'foo'
    E474: 'context' requires a number
    E474: 'ver' number is out of range
    E474: 'algorithm' must be one of: myers, minimal, patience, histogram
    E474: 'filler' does not take a value

Related:

- #31084
- #34661
- #31820
- #14739
- #20107
- fix #18875
  - :get() returns `{ sbr = true, shift = '3' }` (reified-keyset) instead of `{'sbr', 'shift:3'}`
  - Setting via table now works too. `object_as_optval_for` `is_map` now recognizes struct options.
- fix #30296
  - instead of `E474: Invalid argument`, errors now look like:
    ```
    E474: Invalid value 'x', expected one of: single, double: ambiwidth=x
    E474: Unknown item 'foo': diffopt=foo
    E474: 'context' requires a number: diffopt=context:x
    ```

simplify `win_float_parse_option` from #26799.

Although `nlua_call_excmd` is semantically for implementing Ex-commands,
the `require()` should never fail, so that's a "Lua error".

But if the call itself fails (the later `semsg` call), that's an "Ex
cmd" error.

Problem:
Too much boilerplate needed to use Lua to impl an excmd or f_xx
function.

Solution:
- Add `nlua_call_vimfn` which takes the args typval, executes
  Lua, and returns a typval.
- refactor(excmd): lua impl for :log, :lsp

Problem:
- Choosing a new EXX error code is tedious.
- It's possible to accidentally use an EXX error code for different
  purposes.

Solution:
Add a lint check which requires EXX error codes to have a :help tag.
This also avoids duplicates because `make doc` does `:helptags ++t doc`
which fails if duplicates are found.