Fix tokenizer crash

This would crash from the highlighter for something like

`PATH={$PATH[echo " "`

The underlying cause is that we use "char_at" which panics on
overread.

So instead this implements try_char_at and then just returns None.

tests/checks/indent.fish

@@ -432,3 +432,6 @@ echo 'begin
 echo 'multiline-\\
 -word' | $fish_indent --check
 echo $status #CHECK: 0
+
+echo 'PATH={$PATH[echo " "' | $fish_indent --ansi
+# CHECK: PATH={$PATH[echo " "