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`.

helpers.source() was a hack to work around the lack of anonymous
:source. Its "create tempfile" behavior is not a required part of most
tests that use it.

Some tests still need the old "create tempfile" behavior either because
they test SID behavior, or because of missing nvim_exec features: #16071

Works similar to ex <f-args>. It only splits the arguments if the
command has more than one posible argument. In cases were the command
can only have 1 argument opts.fargs = { opts.args }

This uses the same validation used when defining commands with
`:command`.

Problem:    "verbose set efm" reports the location of the :compiler command.
            (Gary Johnson)
Solution:   Add the "-keepscript" argument to :command and use it when
            defining CompilerSet.
https://github.com/vim/vim/commit/58ef8a31d7087d495ab1582be5b7a22796ac2451