Problem:

Whether an option is allowed to be empty isn't well defined and
isn't properly checked.

Solution:

- For non-list string options, explicitly check the option value
  if it is empty.
- Annotate non-list string options that can accept an empty value.
  - Adjust command completion to ignore the empty value.
- Render values in Lua meta files

Problem:
The way option scopes currently work is inflexible and does not allow for nested
option scopes or easily finding the value of an option at any arbitrary scope
without having to do long handwritten switch-case statements like in
`get_varp()`. `.indir` is also confusing and redundant since option indices for
each scope can be autogenerated.

Solution:
Expand option scopes in such a way that an option can support any amount of
scopes using a set of scope flags, similarly to how it's already done for option
types. Also make options contain information about its index at each scope it
supports. This allows for massively simplifying `get_varp()` and
`get_varp_scope()` in the future by just using a struct for options at each
scope. This would be done by creating a table that stores the offset of an
option's variable at a scope by using the option's index at that scope as a key.
This PR also autogenerates enums for option indices at each scope to remove the
need for `.indir` entirely, and also to allow easily iterating over options all
options that support any scope.

Ref: #29314

Problem:
There are three different ways of marking an option as hidden, `enable_if
= false`, `hidden = true` and `immutable = true`. These also have different
behaviors. Options hidden with `enable_if = false` can't have their value
fetched using Vim script or the API, but options hidden with `hidden = true` or
`immutable = true` can. On the other hand, options with `hidden = true` do not
error when trying to set their value, but options with `immutable = true` do.

Solution:
Remove `enable_if = false`, remove the `hidden` property for options, and use
`immutable = true` to mark an option as hidden instead. Also make hidden option
variable pointers always point to the default value, which allows fetching the
value of every hidden option using Vim script and the API. This does also mean
that trying to set a hidden option will now give an error instead of just being
ignored.

Problem:
Hidden options are documented despite being no-ops.

Solution:
Remove docs for hidden options.
Move tags for options that we plan to restore, to ":help nvim-missing".
Move tags for permanently removed options, to ":help nvim-removed".

Problem:  testing of options can be further improved
Solution: split the generated option test into test_options_all.vim,
          add more test cases, save and restore values, fix use-after-free

closes: vim/vim#15894

https://github.com/vim/vim/commit/6eca04e9f1d446dc509ba51e32da56fa413fe2f0

Co-authored-by: Milly <milly.ca@gmail.com>