lib/types: add missing nvim_set_hl attrs - #4540

Declare the attributes that Neovim 0.12.4's pinned `api.txt` lists for
`nvim_set_hl` but the highlight submodule did not: `fg_indexed`,
`bg_indexed`, `blink`, `overline`, `dim`, `conceal`, `altfont`,
`link_global`, `force`, and `update`.

The submodule is freeform, so these keys already reached Neovim. What
they gain is documentation, so a user can find them without reading
`api.txt`. Booleans use `maybeRaw` and `link_global` follows `link`, so
declaring a key takes away no value shape that `nvim_set_hl` accepts.
Declaring does narrow against the bare freeform type: `force = 1`
evaluated before and now fails, and the keyset agrees it was never valid.

The highlight test exercises these as a realistic configuration. Because
freeform keeps accepting the same values, that alone cannot detect a
removed declaration, so a second case asserts the declarations directly
through `types.highlight.getSubOptions`.

Scope the descriptions to how Nixvim applies highlights. `force` only
matters together with `default`, since `nvim_set_hl` already replaces a
group otherwise. `update` needs the group to exist, so it belongs in
`highlightOverride`, which runs after `colorscheme`. `link_global`
behaves as `link` here, because Nixvim always applies at `ns=0`.