New helper functions __fish_is_first_arg and __fish_prev_arg_in

For usage in completion scripts.

Unlike `__fish_is_first_token` (which is probably not correctly named),
`__fish_is_first_arg` returns true regardless of whether existing tokens start with `-`
or not, to be used when an arg cannot be used with any other argument.

`__fish_prev_arg_in` is similar to `__fish_seen_...` but it explicitly
tests the preceding token only, for arguments that take only a single
parameter.

share/functions/__fish_prev_arg_in.fish

@@ -0,0 +1,16 @@
+# returns 0 only if previous argument is one of the supplied arguments
+function __fish_prev_arg_in
+	set -l tokens (commandline -co)
+	set -l tokenCount (count $tokens)
+	if test $tokenCount -lt 2
+		# need at least cmd and prev argument
+		return 1
+	end
+	for arg in $argv
+		if string match -q -- $tokens[-1] $arg
+			return 0
+		end
+	end
+
+	return 1
+end