commandline: Add --is-valid option (#8142)

* commandline: Add --is-valid option to query whether it's syntactically complete

This means querying when the commandline is in a state that it could
be executed. Because our `execute` bind function also inserts a
newline if it isn't.

One case that's not handled right now: `execute` also expands
abbreviations, those can technically make the commandline invalid
again.

Unfortunately we have no real way to *check* without doing the
replacement.

Also since abbreviations are only available in command position when
you _execute_ them the commandline will most likely be valid.

This is enough to make transient prompts work:

```fish
function reset-transient --on-event fish_postexec
    set -g TRANSIENT 0
end

function maybe_execute
    if commandline --is-valid
        set -g TRANSIENT 1
        commandline -f repaint
    else
        set -g TRANSIENT 0
    end
    commandline -f execute
end

bind \r maybe_execute
```

and then in `fish_prompt` react to $TRANSIENT being set to 1.

doc_src/cmds/commandline.rst

@@ -61,6 +61,7 @@ The following options output metadata about the commandline state:
 
 - ``-P`` or ``--paging-mode`` evaluates to true if the commandline is showing pager contents, such as tab completions
 
+- ``--is-valid`` returns true when the commandline is syntactically valid and complete. If it is, it would be executed when the ``execute`` bind function is called. If the commandline is incomplete, it returns 2, if it is erroneus, it returns 1.
 
 Example
 -------