input: Disallow `table/chain=` and make `chain` apply to the most recent table

[mitchellh]

Currently, this is a footgun, and I already had two separate users hit it:

keybind = ctrl+n=activate_key_table:foo
keybind = foo/a=text:hello
keybind = chain=deactivate_key_table

What do you think this should do?

Today: the chain applies to ctrl+n because it is scoped to the set it is modifying. The result is that you can never enter the foo table anymore (because ctrl+n activates then immediately deactivates the table).

I think this is counterintuitive.

I propose that chain is only allowed at the root level (table/chain= is disallowed) and it applies to whatever the most recent binding was in any table. The above example would then apply the chain to foo/.