Adds a `name` key to the opts dict passed to Lua command callbacks
created using `nvim_create_user_command()`. This is useful for when
multiple commands use the same callback.

Note that this kind of behavior is not as strange as one might think,
even some internal Neovim commands reuse the same internal C function,
differing their behavior by checking the command name. `substitute`,
`smagic` and `snomagic` are examples of that.

This will also be useful for generalized Lua command preview functions
that can preview a wide range of commands, in which case knowing the
command name is necessary for the preview function to actually be able
to execute the command that it's supposed to preview.

I don't think using an integer as a NUL-terminated string can work on
big-endian systems, at least.

This is also not tested. Add a test.

Also fix a mistake in the docs of nvim_parse_cmd.

Now nvim_parse_cmd and nvim_create_user_command use a "tab" value which
is the same as the number passed before :tab modifier instead of the
number plus 1, and "tab" value is -1 if :tab modifier is not used.

Problem: A command defined with `nargs="?"` returns `fargs={""}` to
a Lua callback when executed with no arguments, which is inconsistent
with how`nargs="*"` behaves.

Solution: Pass `fargs={}` for no argument with `nargs="?"` as well.

Closes #18876.

Adds a Lua-only `preview` flag to user commands which allows the command to be incrementally previewed like `:substitute` when 'inccommand' is set.

Adds an `smods` key to `nvim_create_user_command` Lua command callbacks,
which has command modifiers but in a structured format. This removes the
need to manually parse command modifiers. It also reduces friction in
using `nvim_cmd` inside a Lua command callback.