Add option(s) to disable autoclosing of parens, quotes, blocks, etc.

[DeedleFake]

I might be in the minority on this, but I find the autoclosing of parentheses and quotes, and to some extent even do end blocks, to be very difficult to work with. An option to disable those features would be very much appreciated.

I'm not that familiar with Livebook's codebase, but I took a quick look to see how this would be implemented and I can try to put a pull request together if the maintainers agree with it being added.

[jonatanklosko]

A PR to add an option for automatic bracket-closing is welcome. This would go in the Settings page, under "Code editor", you can see similar options for how it is handled.

Note that the do end behaviour separate, defined by the language plugin, and I think making this configurable is too much.