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.

Skip runs of whitespace and do not include `\` characters when
followed by another `\` or whitespace. This matches the behavior
of <f-args> when used with `:command`.