fish completions: fix double-evaluation of commandline

We capture the commandline tokens using fish's "commandline --tokenize" (-o).
Given a commandline of

	some-cobra-command 'some argument $(123)' "$HOME"

into three arguments while removing the quotes

	some-cobra-command
	some argument $(123)
	$HOME

Later we pass "some argument $(123)" without quotes to the shell's
"eval". This is wrong and causes spurious evaluation of the parenthesis
as command substitution.

Fix this by escaping the arguments.

The upcoming fish 3.8 has a new flag, "commandline -x"
which will expand variables like $HOME properly, see
https://github.com/fish-shell/fish-shell/pull/10212 Use that if
available.

Reproduce the issue by pasting the completion script at
https://github.com/fish-shell/fish-shell/issues/10194#issuecomment-1879563545
to a file "grafana-manager.fish" and running

	function grafana-manager; end
	source grafana-manager.fish

Then type (without pressing Enter)

	grafana-manager public-dashboards delete --organization-id 3 --dashboard-name "k8s (public)" <TAB>

Fixes https://github.com/fish-shell/fish-shell/issues/10194

fish_completions.go

@@ -45,7 +45,9 @@ function __%[1]s_perform_completion
     __%[1]s_debug "Starting __%[1]s_perform_completion"
 
     # Extract all args except the last one
-    set -l args (commandline -opc)
+    set -l args (
+	    commandline -xpc 2>/dev/null ||
+	    commandline -opc | string escape)
     # Extract the last arg and escape it in case it is a space
     set -l lastArg (string escape -- (commandline -ct))